CoStar (CSGP) 10-K 23 Feb 18 2017 FY Annual report Financial data

DOCUMENT AND ENTITY INFORMATION

DOCUMENT AND ENTITY INFORMATION - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Feb. 16, 2018	Jun. 30, 2017
Document and Entity Information [Abstract]
Entity Registrant Name	COSTAR GROUP INC
Entity Central Index Key	1,057,352
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 8
Entity Common Stock, Shares Outstanding (in shares)		36,094,701
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenues	$ 965,230	$ 837,630	$ 711,764
Cost of revenues	220,403	173,814	188,885
Gross profit	744,827	663,816	522,879
Operating expenses:
Selling and marketing (excluding customer base amortization)	318,362	296,483	302,226
Software development	88,850	76,400	65,760
General and administrative	146,128	123,297	115,507
Customer base amortization	17,671	22,731	27,931
Total operating expenses	571,011	518,911	511,424
Income from operations	173,816	144,905	11,455
Interest and other income	4,044	1,773	537
Interest and other expense	(9,014)	(10,016)	(9,411)
Loss on debt extinguishment	(3,788)	0	0
Income before income taxes	165,058	136,662	2,581
Income tax expense	42,363	51,591	6,046
Net income (loss)	$ 122,695	$ 85,071	$ (3,465)
Net income (loss) per share — basic (in dollars per share)	$ 3.70	$ 2.64	$ (0.11)
Net income (loss) per share — diluted (in dollars per share)	$ 3.66	$ 2.62	$ (0.11)
Weighted average outstanding shares — basic (in shares)	33,200	32,167	31,950
Weighted average outstanding shares — diluted (in shares)	33,559	32,436	31,950

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 122,695	$ 85,071	$ (3,465)
Other comprehensive loss, net of tax
Foreign currency translation adjustment	3,901	(5,032)	(1,466)
Net decrease in unrealized loss on investments	118	395	256
Reclassification adjustment for realized gains on investments included in net income	0	(808)	0
Total other comprehensive income (loss)	4,019	(5,445)	(1,210)
Total comprehensive income (loss)	$ 126,714	$ 79,626	$ (4,675)

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 1,211,463	$ 567,223
Accounts receivable, net of allowance for doubtful accounts of approximately $6,469 and $6,344 as of December 31, 2017 and 2016, respectively	60,900	48,537
Income tax receivable	0	129
Prepaid expenses and other current assets	15,572	11,602
Total current assets	1,287,935	627,491
Long-term investments	10,070	9,952
Deferred income taxes, net	5,431	7,273
Property and equipment, net	84,496	87,568
Goodwill	1,283,457	1,254,866
Intangible assets, net	182,892	195,965
Deposits and other assets	6,179	1,948
Income tax receivable	12,981	0
Total assets	2,873,441	2,185,063
Current liabilities:
Current portion of long-term debt	0	31,866
Accounts payable	9,262	11,478
Accrued wages and commissions	54,104	33,803
Accrued expenses	22,193	31,092
Deferred gain on the sale of building	2,523	2,523
Income taxes payable	8,166	3,814
Deferred rent	4,732	1,206
Deferred revenue	45,686	39,164
Total current liabilities	146,666	154,946
Long-term debt, less current portion	0	306,473
Deferred gain on the sale of building	16,192	18,715
Deferred rent	33,909	31,589
Deferred income taxes, net	12,070	18,386
Income taxes payable	13,354	741
Total liabilities	222,191	530,850
Commitments and contingencies (Note 10)
Stockholders’ equity:
Preferred stock, $0.01 par value; 2,000 shares authorized; none outstanding	0	0
Common stock, $0.01 par value; 60,000 shares authorized; 36,107 and 32,606 issued and outstanding as of December 31, 2017 and 2016, respectively	361	326
Additional paid-in capital	2,339,253	1,471,127
Accumulated other comprehensive loss	(9,020)	(13,039)
Retained earnings	320,656	195,799
Total stockholders’ equity	2,651,250	1,654,213
Total liabilities and stockholders’ equity	$ 2,873,441	$ 2,185,063

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts	$ 6,344	$ 7,478
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	2,000,000	2,000,000
Preferred stock, shares outstanding (in shares)	0	0
Common stock par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	60,000,000	60,000,000
Common stock, shares issued, (in shares)	32,606,000	32,509,000
Common stock, shares outstanding (in shares)	32,606,000	32,509,000

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Loss	Retained Earnings
Balance (in shares) at Dec. 31, 2014		32,318
Balance at Dec. 31, 2014	$ 1,513,546	$ 323	$ 1,405,414	$ (6,384)	$ 114,193
Net income (loss)	(3,465)				(3,465)
Other comprehensive loss	(1,210)			(1,210)
Exercise of stock options (in shares)		60
Exercise of stock options	5,069	$ 1	5,068
Restricted stock grants (in shares)		239
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(121)
Restricted stock grants surrendered	(16,436)	$ (1)	(16,435)
Stock compensation expense, net of forfeitures	35,153		35,153
Employee stock purchase plan (in shares)		13
Employee stock purchase plan	2,595	$ 0	2,595
Excess tax benefit from stock-based compensation	8,528		8,528
Balance (in shares) at Dec. 31, 2015		32,509
Balance at Dec. 31, 2015	1,543,780	$ 325	1,440,321	(7,594)	110,728
Net income (loss)	85,071				85,071
Other comprehensive loss	(5,445)			(5,445)
Exercise of stock options (in shares)		29
Exercise of stock options	3,303	$ 0	3,303
Restricted stock grants (in shares)		195
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(142)
Restricted stock grants surrendered	(16,424)	$ (1)	(16,423)
Stock compensation expense, net of forfeitures	36,388		36,388
Employee stock purchase plan (in shares)		15
Employee stock purchase plan	2,842	$ 0	2,842
Excess tax benefit from stock-based compensation	4,698		4,698
Balance (in shares) at Dec. 31, 2016		32,606
Balance at Dec. 31, 2016	1,654,213	$ 326	1,471,127	(13,039)	195,799
Net income (loss)	122,695				122,695
Other comprehensive loss	4,019			4,019
Exercise of stock options (in shares)		82
Exercise of stock options	6,797	$ 1	6,796
Restricted stock grants (in shares)		187
Restricted stock grants	0	$ 2
Restricted stock grants adjustments			(2)
Restricted stock grants surrendered (in shares)		(99)
Restricted stock grants surrendered	(14,902)	$ (1)	(14,901)
Stock compensation expense, net of forfeitures	38,921		38,921
Stock issued for equity offering (in shares)		3,317
Stock issued for equity offering	833,911	$ 33	833,878
Employee stock purchase plan (in shares)		14
Employee stock purchase plan	3,434	$ 0	3,434
Balance (in shares) at Dec. 31, 2017		36,107
Balance at Dec. 31, 2017	2,651,250	$ 361	$ 2,339,253	$ (9,020)	320,656
Cumulative effect of adoption of new accounting standard	$ 2,162				$ 2,162

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net income (loss)	$ 122,695	$ 85,071	$ (3,465)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	63,643	70,165	78,532
Amortization of debt issuance costs	2,303	3,227	3,311
Loss on extinguishment of debt	3,788	0	0
Impairment loss	0	23	2,778
Loss on disposal of property and equipment	129	839	681
Realized gain on investments	0	(808)	0
Stock-based compensation expense	39,030	36,349	34,537
Deferred income tax (benefit) expense, net	(2,903)	15,635	(5,792)
Bad debt expense	5,690	7,358	7,002
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	(17,524)	(16,044)	(3,999)
Income taxes payable	16,937	2,816	11,380
Prepaid expenses and other current assets	(3,672)	(1,157)	367
Income tax receivable	(12,981)	0	0
Deposits and other assets	39	758	686
Accounts payable and other liabilities	11,525	(1,520)	9,938
Deferred revenue	6,004	(2,070)	3,817
Net cash provided by operating activities	234,703	200,642	139,773
Investing activities:
Proceeds from sale and settlement of investments	0	5,950	1,900
Purchases of property and equipment and other assets	(24,499)	(18,766)	(35,061)
Acquisitions, net of cash acquired	(47,768)	(10,443)	(182,341)
Net cash used in investing activities	(72,267)	(23,259)	(215,502)
Financing activities:
Payments of long-term debt	(345,000)	(20,000)	(20,000)
Payments of debt issuance costs	(3,467)	0	0
Repurchase of restricted stock to satisfy tax withholding obligations	(14,902)	(16,424)	(16,436)
Proceeds from equity offering, net of transaction costs	833,911	0	0
Proceeds from exercise of stock options and employee stock purchase plan	9,888	5,861	7,404
Net cash provided by (used in) financing activities	480,430	(30,563)	(29,032)
Effect of foreign currency exchange rates on cash and cash equivalents	1,374	(1,415)	(433)
Net increase (decrease) in cash and cash equivalents	644,240	145,405	(105,194)
Cash and cash equivalents at beginning of year	567,223	421,818	527,012
Cash and cash equivalents at end of year	$ 1,211,463	$ 567,223	$ 421,818

ORGANIZATION

ORGANIZATION	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
ORGANIZATION	ORGANIZATION CoStar Group, Inc. (the “Company” or “CoStar”) provides information, analytics and online marketplace services to the commercial real estate and related business community through its comprehensive, proprietary database of commercial real estate information covering the United States (“U.S.”), the United Kingdom (“U.K.”), and parts of Canada, Spain, Germany and France. The Company provides online marketplaces for commercial real estate, apartment rentals, lands for sale and businesses for sale. The Company operates within two operating segments, North America and International, and its services are typically distributed to its clients under subscription-based license agreements that renew automatically, a majority of which have a term of one year .

SUMMARY OF SIGNIFICANT ACCOUNTI

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Accounting policies are consistent for each operating segment. Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to revenue recognition, allowance for doubtful accounts, useful lives of property and equipment and intangible assets, recoverability of long-lived assets and intangible assets with definite lives, goodwill, income taxes, fair value of equity instruments, fair value of auction rate securities (“ARS”), accounting for business combinations and contingencies, among others. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that it believes are reasonable, including assumptions as to future events. These estimates form the basis for making judgments about the carrying values of assets and liabilities and recorded revenues and expenses. Actual results could differ from these estimates. Revenue Recognition The Company derives revenues by providing access to its proprietary database of commercial real estate information, typically through a fixed monthly fee for its subscription-based services. The Company's subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography, the number and types of services to which a client subscribes, the number of properties a client advertises and the prominence and placement of a client's advertised properties in the search results. A majority of the subscription-based license agreements have a term of one year and renew automatically. Revenues are recognized when (1) there is persuasive evidence of an arrangement, (2) the fee is fixed or determinable, (3) services have been rendered and payment has been contractually earned and (4) collectability is reasonably assured. Revenues from subscription-based services are recognized on a straight-line basis over the term of the agreement. Deferred revenue results from advance cash receipts from customers or amounts billed in advance to customers from the sale of subscription licenses and is recognized over the term of the license agreement. 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Revenue Recognition — (Continued) The Company analyzes contracts with multiple elements under the accounting guidance for multiple-element arrangements. The Company's multiple-element arrangements include information, analytics and/or online marketplace services that are generally provided to the customer over the same term. When identifying multiple-element arrangements, the Company considers multiple purchases made by the same customer within a short time frame and assesses whether the purchases were negotiated together as one overall arrangement. If a multiple-element arrangement is identified, then the arrangement consideration is allocated among the separate units of accounting based on their relative selling prices, which are estimated considering factors such as historical pricing, pricing strategy, market conditions and other factors. The Company accounts for each deliverable in the transaction separately. If the deliverables cannot be separated into multiple units of accounting, then the arrangement consideration is combined and recognition of revenue is determined for the combined unit of accounting. Multiple-element transactions require judgment to determine the selling price or fair value of the different elements. These judgments impact the amount of revenue recognized over the term of the contract, as well as the period in which they are recognized. Cost of Revenues Cost of revenues principally consists of salaries, benefits, bonuses, and stock-based compensation expenses for the Company’s researchers who collect and analyze the commercial real estate data that is the basis for the Company’s information, analytics and online marketplaces. Additionally, cost of revenues includes the cost of data from third-party data sources, credit card and other transaction fees relating to processing customer transactions, which are expensed as incurred, and the amortization of acquired trade names and other intangible assets and database technology. Foreign Currency Translation The Company’s functional currency in its foreign locations is the local currency. Assets and liabilities are translated into U.S. dollars using the exchange rates as of the balance sheet dates. Revenues, expenses, gains and losses are translated at the average exchange rates in effect during each period. Gains and losses resulting from translation are included in accumulated other comprehensive loss. Currency g ains and losses on the translation of intercompany loans made to foreign subsidiaries that are of a long-term investment nature are also included in accumulated other comprehensive loss. Net gains or losses resulting from foreign currency exchange transactions are included in the consolidated statements of operations. There were no material gains or losses from foreign currency exchange transactions for the years ended December 31, 2017 , 2016 , and 2015 . Accumulated Other Comprehensive Loss The components of accumulated other comprehensive loss were as follows (in thousands): As of December 31, 2017 2016 Foreign currency translation adjustment $ (8,290 ) $ (12,191 ) Net unrealized loss on investments, net of tax (730 ) (848 ) Total accumulated other comprehensive loss $ (9,020 ) $ (13,039 ) There were no amounts reclassified out of accumulated other comprehensive loss to the consolidated statement of operations for the year ended December 31, 2017 . The amount of realized gain from the redemption of available-for-sale securities reclassified out of accumulated other comprehensive loss to the consolidated statement of operations for the year ended December 31, 2016 was approximately $808,000 . There were no amounts reclassified out of accumulated other comprehensive loss to the consolidated statements of operations for the year ended December 31, 2015 . Advertising Costs The Company expenses advertising costs as incurred. Advertising costs include e-commerce, television, radio, print and other media advertising. Advertising costs were approximately $104 million , $109 million and $132 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Income Taxes Deferred income taxes result from temporary differences between the tax basis of assets and liabilities and the basis reported in the Company’s consolidated financial statements. Deferred tax liabilities and assets are determined based on the difference between the financial statement and the tax basis of assets and liabilities using enacted rates in effect during the year in which the Company expects differences to reverse. Valuation allowances are provided against assets, including net operating losses, if the Company anticipates that some or all of an asset may not be realized through future taxable earnings or implementation of tax planning strategies. Interest and penalties related to income tax matters are recognized in income tax expense. See Note 9 for additional information regarding income taxes. Net Income (Loss) Per Share Net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period on a basic and diluted basis. The Company’s potentially dilutive securities include stock options and restricted stock. Diluted net income (loss) per share considers the impact of potentially dilutive securities except in periods in which there is a net loss, as the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Stock-Based Compensation Equity instruments issued in exchange for services performed by officers, employees, and directors of the Company are accounted for using a fair-value based method and the fair value of such equity instruments is recognized as expense in the consolidated statements of operations. Stock-based compensation expense is measured at the grant date of the stock-based awards that vest over set time periods based on their fair values, and is recognized on a straight-line basis as expense over the vesting periods of the awards, net of an estimated forfeiture rate. For equity instruments that vest based on performance, the Company assesses the probability of the achievement of the performance conditions at the end of each reporting period, or more frequently based upon the occurrence of events that may change the probability of whether the performance conditions would be met. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing of recognition may fluctuate from period to period based on those estimates. For equity instruments that vest based on a performance condition and a market condition, the Company estimates the fair value of each equity instrument granted on the date of grant using a Monte-Carlo simulation model. This pricing model uses multiple simulations to evaluate the probability of achieving the market condition to calculate the fair value of the awards. Stock-based compensation expense is updated based on the expected achievement of the related performance conditions at the end of each reporting period. If the performance conditions are not met, no stock-based compensation expense will be recognized, and any previously recognized stock-based compensation expense will be reversed. In March 2017, March 2016 and March 2015, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the Company’s achievement of a three-year cumulative revenue goal established at the grant date, and are subject to forfeiture in the event the foregoing performance condition is not met by the end of each respective three-year period. These grants of restricted common stock are also subject to continuing employment requirements and a market condition based on total shareholder return (“TSR”). The actual number of shares that vest at the end of each respective three-year period is determined based on the Company’s achievement of the three -year performance goals described above, as well as its TSR relative to the Russell 1000 Index over the same three-year performance period. Each reporting period, the Company reassesses the probability of achieving the performance and market conditions and determines whether it is probable that the performance and market conditions for the awards would be met. The Company recorded a total of approximately $5 million , $3 million and $3 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the years ended December 31, 2017 , 2016 , and 2015 , respectively. The Company expects to record estimated stock-based compensation expense related to the performance-based restricted common stock awards of approximately $6 million in aggregate over the periods 2018, 2019 and 2020 . 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Stock-Based Compensation — (Continued) Stock-based compensation expense for stock options and restricted stock issued under equity incentive plans and stock purchases under the ESPP included in the Company’s results of operations were as follows (in thousands): Year Ended December 31, 2017 2016 2015 Cost of revenues $ 4,971 $ 5,495 $ 5,815 Selling and marketing (excluding customer base amortization) 7,086 6,634 5,114 Software development 7,071 6,546 5,712 General and administrative 19,902 17,674 17,896 Total stock-based compensation $ 39,030 $ 36,349 $ 34,537 Cash and Cash Equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents . Cash equivalents consist of money markets and commercial paper. Investments The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities. Investments are carried at fair value. Changes in unrealized holding gains and losses, net of the related tax effect, on available-for-sale securities are excluded from earnings and are reported as a separate component of accumulated other comprehensive loss in stockholders’ equity until realized. A decline in market value of any available-for-sale security below cost that is deemed to be other-than-temporary results in a reduction in carrying amount to fair value. The impairment is charged to earnings and a new cost basis for the security is established. Dividend and interest income are recognized when earned. Concentration of Credit Risk and Financial Instruments The Company performs ongoing assessments of its customers’ financial conditions and generally does not require that its customers’ obligations to the Company be secured. The Company maintains reserves for estimated inherent credit losses, and such losses have been within management’s expectations. The large size and widespread nature of the Company’s customer base and the Company’s lack of dependence on any individual customer mitigates the risk of nonpayment of the Company’s accounts receivable. No single customer accounted for more than 5% of the Company’s revenues for each of the years ended December 31, 2017, 2016, and 2015. The carrying amount of the accounts receivable approximates the net realizable value. The Company holds cash at major financial institutions that often exceed Federal Deposit Insurance Corporation insured limits. The Company manages its credit risk associated with cash concentrations by concentrating its cash deposits in high quality financial institutions and by periodically evaluating the credit quality of the primary financial institutions holding such deposits. The carrying value of cash approximates fair value. Historically, the Company has not experienced any losses due to such cash concentrations. 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Accounts Receivable, Net of Allowance for Doubtful Accounts Accounts receivable are recorded at the invoiced amount net of credits due. Accounts receivable payment terms vary and amounts due from customers are stated in the financial statements net of an allowance for doubtful accounts. When evaluating the adequacy of the allowance for doubtful accounts, the Company analyzes historical collection experience, changes in customer payment profiles and the aging of receivable balances, as well as current economic conditions, all of which may affect a customer’s ability to pay. Property and Equipment, Net Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets: Leasehold improvements Shorter of lease term or useful life Furniture and office equipment Five to ten years Vehicles Five to ten years Computer hardware and software Two to five years Qualifying internal-use software costs incurred during the application development stage, which consist primarily of internal product development costs, outside services and purchased software license costs are capitalized and amortized over the estimated useful life of the asset. All other costs are expensed as incurred. Goodwill and Intangible Assets Goodwill represents the excess of costs over the fair value of assets of acquired businesses. Goodwill is not amortized, but instead tested for impairment at least annually by each reporting unit. The Company may first assess qualitative factors to evaluate whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount or elect to bypass such assessment. If it is determined that it is more likely than not that the fair value of a reporting unit is less than its carrying value, or the Company elects to bypass such assessment, the Company then determines the fair value of each reporting unit. The estimate of the fair value of each reporting unit is based on a projected discounted cash flow model that includes significant assumptions and estimates including the Company's discount rate, growth rate and future financial performance. Assumptions about the discount rate are based on a weighted average cost of capital for comparable companies. Assumptions about the growth rate and future financial performance of a reporting unit are based on the Company's forecasts, business plans, economic projections and anticipated future cash flows. The fair value of each reporting unit is compared to the carrying amount of the reporting unit. If the carrying value of the reporting unit exceeds the fair value, then an impairment loss is recognized for the difference. Acquired database technology, customer base and trade names and other intangible assets are related to the Company’s acquisitions (see Notes 6 and 7 ). Acquired database technology is amortized on a straight-line basis over periods ranging from three years to eight years . Acquired trade names and other intangible assets are amortized on a straight-line basis over periods ranging from three years to fifteen years . See Note 7 for further details on the reclassification of the acquired trade names recorded in connection with the LoopNet acquisition from an indefinite-lived intangible asset to a definite-lived intangible asset. Acquired intangible assets characterized as customer base consists of acquired customer contracts and the related customer relationships and are amortized over periods ranging from ten years to thirteen years . Acquired customer bases are typically amortized on an accelerated basis related to the expected economic benefit of the intangible asset. The cost of capitalized building photography is amortized on a straight-line basis over periods ranging from three years to five years . Intangible assets are reviewed for impairment at least annually, and more frequently whenever events or changes in circumstances indicate that the carrying value may not be recoverable . 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Long-Lived Assets Long-lived assets, such as property and equipment, and purchased intangibles subject to amortization, are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount of an asset exceeds its estimated future cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset. Assets to be disposed of would be separately presented in the balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and would no longer be depreciated. The assets and liabilities of a disposal group classified as held for sale would be presented separately in the appropriate asset and liability sections of the balance sheet. Debt Issuance Costs Costs incurred in connection with the issuance of long-term debt are deferred and amortized as interest expense over the term of the related debt using the effective interest method for term debt and on a straight-line basis for revolving debt. These amounts are reflected in the consolidated balance sheets as direct deductions from a combination of the current and long-term portions of debt for term debt and as current and long-term assets for costs related to revolving debt. Upon a refinancing or amendment, previously capitalized debt issuance costs are expensed and included in loss on extinguishment of debt if the Company determines that there has been a substantial modification of the related debt. If the Company determines that there has not been a substantial modification of the related debt, any previously capitalized debt issuance costs are amortized as interest expense over the term of the new debt instrument. The Company had capitalized debt issuance costs, net of amortization, of approximately $4 million and $7 million as of December 31, 2017 and 2016 , respectively. The debt issuance costs are associated with our various previous credit agreements, and the current amended and restated 2017 Credit Agreement (the "2017 Credit Agreement"). See Note 8 for additional information regarding the term loan and revolving credit facility. The Company recognized debt issuance costs of approximately $2 million , $3 million , and $3 million and included in interest expense for each of the years ended December 31, 2017 , 2016 , and 2015 . Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, and acquired trade names from a market participant's perspective, useful lives and discount rates. During the measurement period, the Company may record adjustments to the assets acquired and liabilities assumed. Any adjustments to provisional amounts that are identified during the measurement period are recorded in the reporting period in which the adjustment amounts are determined. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings. Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In March 2016, the FASB issued ASU 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting , which simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification of share-based payment transactions on the statement of cash flows. The guidance requires a company to (i) recognize all excess tax benefits and tax deficiencies as income tax expense or benefit in the statement of operations using a prospective transition method, (ii) recognize excess tax benefits in the current period regardless of whether the benefit reduces taxes payable using a modified retrospective transition method, and (iii) classify all excess tax benefits as operating activities within the statement of cash flows using either a prospective transition method or a retrospective transition method. The guidance also allows a company to (i) elect whether to estimate the number of share-based awards expected to vest or account for forfeitures when they occur, and (ii) withhold up to the maximum statutory tax rate in the applicable jurisdiction for awards, both of which should be applied using a modified retrospective transition method. Finally, the guidance requires a company to classify the cash paid by an employer when directly withholding shares for tax withholding purposes as a financing activity within the statement of cash flows using a retrospective transition method. This guidance is effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. The Company adopted the new guidance on January 1, 2017. The adoption of the new standard resulted in a $2 million cumulative-effect adjustment as of January 1, 2017 to record a deferred tax asset with the offset to retained earnings on the balance sheet, representing the amount of the Company's net operating loss carryforwards attributable to excess tax benefits that it was not able to record under the prior guidance. The Company elected to continue to estimate the number of awards expected to be forfeited and adjust the estimate when it is no longer probable that the service or performance conditions will be met. Additionally, the Company elected to apply the presentation requirements for statement of cash flows related to excess tax benefits retrospectively to all periods presented, which resulted in an increase to both net cash provided by operating activities and net cash used in financing activities of approximately $5 million and $9 million for the twelve months ended December 31, 2016 and 2015 , respectively. The presentation requirements related to employee taxes paid by withholding shares had no impact on any of the periods presented in the Company's consolidated cash flows statements since such cash flows have historically been presented as a financing activity. In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350: Simplifying the Test for Goodwill Impairment , which is designed to simplify the subsequent measurement of goodwill by eliminating Step 2 from the goodwill impairment test. The guidance indicates that an entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2019, including interim periods within that reporting period. Early application is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company early adopted this guidance during the first quarter of 2017 and the early adoption did not have a material impact on the Company's consolidated financial statements and related disclosures. Recent Accounting Pronouncements Not Yet Adopted In May 2014, the Financial Accounting Standards Board (“FASB”) and International Accounting Standards Board (“IASB”) jointly issued a new revenue recognition standard, Accounting Standards Update (“ASU) 2014-09, Revenue from Contracts with Customers (Topic 606) that is designed to improve financial reporting by creating common recognition guidance for GAAP and International Financial Reporting Standards (“IFRS”). This guidance provides a robust framework for addressing revenue issues, improves the comparability of revenue recognition practices across industries, provides useful information to users of financial statements through improved disclosure requirements and simplifies the presentation of financial statements. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goo ds or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Subsequently, amendments to the new revenue recognition standard were issued to clarify numerous accounting topics, including, but not limited to (i) the implementation guidance on principal versus agent considerations, (ii) the identification of performance obligations, (iii) the licensing implementation guidance, (iv) the objective of the collectability criterion, (v) the application o f the variable consideration guidance and modified retrospective transition method, (vi) the way in which impairment testing is performed and (vii) the disclosure requirements for revenue recognized from performance obligations. This guidance permits the use of either a full retrospective method or a modified retrospective approach. The modified retrospective approach would be applied only to the most current period presented along with a cumulative-effect adjustment at the date of adoption. This guidance will be effective for annual reporting periods beginnin g after December 15, 2017, although companies may adopt the standard as early as annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. The Company currently intends to adopt the new guidance using the modified retrospective approach. Under this adoption method, the Company will recognize a cumulative effect adjustment to retained earnings as of January 1, 2018 and will account for its contracts with customers under the new guidance prospectively beginning on January 1, 2018. The Company used several available practical expedients provided in the new guidance, including assessing contracts with similar terms and conditions on a “portfolio” basis. Based on the Company’s preliminary analysis, the Company believes that the potential impact of adopting this guidance on reported revenue in any period will not be material. The primary impact of adopting the new guidance relates to the deferral of incremental costs of obtaining customer contracts which is primarily commission costs on new sales. Under existing guidance, the Company expensed all commission costs in the periods they were earned, whereas under the new guidance the Company will defer commission costs on new sales and amortize them on a straight-line basis over three years. The three-year amortization period was determined based on several factors, including the nature of the technology and proprietary data underlying the services being purchased; customer contract renewals rates; and industry competition. The Company currently estimates that the cumulative effect of adoption will result in the recognition of deferred commission cost asset between $48 million to $58 million , net of appropriate taxes, as of January 1, 2018. Beginning in 2018, the Company expects significant changes to its disclosed revenue recognition policies and practices, as well as to other related financial statement disclosures. These revised disclosures will be made in the Company’s first quarterly report in 2018. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) , to increase transparency and comparability among organizations’ accounting for leases. The guidance requires a company to recognize lease assets and lease liabilities on the balance sheet, as well as disclose key information about leasing arrangements. This guidance is effective on a modified retrospective basis for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period. Early application is permitted. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures, but expects that the adoption of this standard may result in a material increase in assets and liabilities on its consolidated balance sheets. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which is designed to provide financial statement users with more information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. When determining such expected credit losses, the guidance requires companies to apply a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. This guidance is effective on a modified retrospective basis for annual report

ACQUISITION

ACQUISITION	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
ACQUISITIONS	ACQUISITION Pursuant to the definitive agreement and plan of merger with Network Communications, Inc. (“NCI”) dated April 27, 2015 (the “Merger Agreement”), on June 1, 2015, the Company acquired 100% of the outstanding stock of NCI and the related Apartment Finder business (collectively referred to as “Apartment Finder”) from the former stockholders of NCI. Apartment Finder provides lead generation, advertising and Internet marketing solutions to property managers and owners through its main service, ApartmentFinder.com TM . The acquisition furthered the Company's expansion into the multifamily vertical. The Apartment Finder acquisition was not deemed to be a significant acquisition. In consideration for the purchase of Apartment Finder, on June 1, 2015, the Company paid approximately $173 million in cash, including an estimated $3 million in connection with a preliminary adjustment for net working capital as of the closing date. Pursuant to the terms of the Merger Agreement, the purchase price was increased by approximately $21,000 following the final determination of the net working capital of NCI as of the closing date, and this amount was paid to NCI in the third quarter of 2015. 3. ACQUISITION — (CONTINUED) The Company applied the acquisition method to account for the Apartment Finder transaction, which requires that, among other things, assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date. The following table summarizes the amounts for acquired assets and liabilities recorded at their fair values as of the acquisition date (in thousands): Cash and cash equivalents $ 39 Accounts receivable 4,556 Goodwill 107,692 Acquired trade names and other intangible assets 23,642 Acquired customer base 21,856 Acquired database technology 4,076 Acquired building photography 2,425 Deferred income taxes, net 9,290 Other assets and liabilities (849 ) Fair value of identifiable net assets acquired $ 172,727 The net assets of Apartment Finder were recorded at their estimated fair value. In valuing acquired assets and liabilities, fair value estimates were based on, but were not limited to, future expected cash flows, market rate assumptions for contractual obligations, and appropriate discount rates. The acquired customer base for the acquisition consisted of three distinct intangible assets, is composed of acquired customer contracts and the related customer relationships, and has a weighted average estimated useful life of ten years . The acquired database technology had an estimated useful life of five months due to the Company's intent to replace the acquired database technology in 2015, which it did in December 2015. The acquired trade names and other intangible assets have a weighted average estimated useful life of nine years . The acquired building photography had an estimated useful life of five months . Amortization of the acquired customer base is recognized on an accelerated basis related to the expected economic benefit of the intangible asset, while amortization of the acquired database technology, acquired building photography and acquired trade names and other intangible assets is recognized on a straight-line basis over their respective estimated useful lives. Goodwill recorded in connection with this acquisition is not amortized, but is subject to annual impairment tests. The approximately $108 million of goodwill recorded as part of the acquisition is associated with the Company's North America operating segment. None of the goodwill recognized is expected to be deductible for income tax purposes in future periods. Goodwill is calculated as the excess of the consideration transferred over the net assets recognized and represents the future economic benefits arising from other assets acquired that could not be individually identified and separately recognized. Specifically, the goodwill recorded as part of the Apartment Finder acquisition includes, but is not limited to: (i) the expected synergies and other benefits that the Company believes will result from combining its operations with Apartment Finder's operations; and (ii) any intangible assets that do not qualify for separate recognition, such as the assembled workforce.

INVESTMENTS

INVESTMENTS	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
INVESTMENTS	INVESTMENTS The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as ARS. Investments are carried at fair value. 3. INVESTMENTS — (CONTINUED) Scheduled maturities of investments classified as available-for-sale as of December 31, 2017 are as follows (in thousands): Maturity Fair Value Due in: 2018 $ — 2019 — 2022 — 2023 — 2027 — 2028 and thereafter 10,070 Available-for-sale investments $ 10,070 The Company had no realized gains on its investments during the year ended December 31, 2017 . The Company realized gains of $0.8 million related to an ARS that was redeemed at a par value of $1 million for the year ended December 31, 2016 . The Company had no realized gains on its investments for the year ended December 31, 2015 . The Company had no realized losses on its investments for the years ended December 31, 2017 , 2016 , and 2015 . Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis. As of December 31, 2017 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 10,800 $ — $ (730 ) $ 10,070 Available-for-sale investments $ 10,800 $ — $ (730 ) $ 10,070 As of December 31, 2016 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 10,800 $ — $ (848 ) $ 9,952 Available-for-sale investments $ 10,800 $ — $ (848 ) $ 9,952 The unrealized losses on the Company’s investments as of December 31, 2017 and 2016 were generated primarily from changes in interest rates and ARS that failed to settle at auction, due to adverse conditions in the global credit markets. The losses are considered temporary, as the contractual terms of these investments do not permit the issuer to settle the security at a price less than the amortized cost of the investment. Because the Company does not intend to sell these instruments and it is not more likely than not that the Company will be required to sell these instruments prior to anticipated recovery, which may be at maturity, the Company does not consider these investments to be other-than-temporarily impaired as of December 31, 2017 and 2016 . See Note 4 for further discussion of the fair value of the Company’s financial assets. 3. INVESTMENTS — (CONTINUED) The components of the Company’s investments in an unrealized loss position for twelve months or longer were as follows (in thousands): December 31, 2017 2016 Aggregate Fair Value Gross Unrealized Losses Aggregate Fair Value Gross Unrealized Losses Auction rate securities $ 10,070 $ (730 ) $ 9,952 $ (848 ) Investments in an unrealized loss position $ 10,070 $ (730 ) $ 9,952 $ (848 ) The Company did not have any investments in an unrealized loss position for less than twelve months as of December 31, 2017 and 2016 , respectively.

FAIR VALUE

FAIR VALUE	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
FAIR VALUE	FAIR VALUE Fair value is defined as the price that would be received in the sale of an asset or paid to transfer a liability in an orderly transaction between market participants. There is a three-tier fair value hierarchy, which categorizes the inputs used in measuring fair value. These tiers include: Level 1, defined as observable inputs such as quoted prices in active markets for identical assets or liabilities; Level 2, defined as inputs other than quoted prices in active markets that are either directly or indirectly observable; and Level 3, defined as unobservable inputs for which little or no market data exists, therefore requiring an entity to develop its own assumptions. The following table represents the Company's fair value hierarchy for its financial assets (cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2017 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money markets $ 586,084 $ — $ — $ 586,084 Commercial paper 351,098 — — 351,098 Auction rate securities — — 10,070 10,070 Total assets measured at fair value $ 937,182 $ — $ 10,070 $ 947,252 The carrying value of accounts receivable, accounts payable, accrued expenses and long-term debt approximates fair value. The following table represents the Company's fair value hierarchy for its financial assets (cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2016 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money markets $ 175,344 $ — $ — $ 175,344 Commercial paper 6,383 — — 6,383 Auction rate securities — — 9,952 9,952 Total assets measured at fair value $ 181,727 $ — $ 9,952 $ 191,679 The Company’s Level 3 assets consist of ARS, whose underlying assets are primarily student loan securities supported by guarantees from the Federal Family Education Loan Program (“FFELP”) of the U.S. Department of Education. 4. FAIR VALUE — (CONTINUED) The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2015 to December 31, 2017 (in thousands): Auction Rate Securities Balance at December 31, 2015 $ 15,507 Decrease in unrealized loss included in accumulated other comprehensive loss 395 Settlements (5,950 ) Balance at December 31, 2016 9,952 Decrease in unrealized loss included in accumulated other comprehensive loss 118 Balance at December 31, 2017 $ 10,070 ARS are variable rate debt instruments whose interest rates are reset approximately every 28 days . The underlying securities have contractual maturities greater than twenty years . The ARS are recorded at fair value. As of December 31, 2017 , the Company held ARS with $11 million par value, all of which failed to settle at auction. The majority of these investments are of high credit quality with AA to AAA credit ratings and are primarily student loan securities supported by guarantees from the FFELP of the U.S. Department of Education. The Company may not be able to liquidate and fully recover the carrying value of the ARS in the near term. As a result, these securities are classified as long-term investments in the Company’s consolidated balance sheet as of December 31, 2017 . See Note 3 for further discussion of the scheduled maturities of investments classified as available-for-sale. While the Company continues to earn interest on its ARS investments at the contractual rate, these investments are not currently actively trading and therefore do not currently have a readily determinable market value. The estimated fair value of the ARS no longer approximates par value. The Company used a discounted cash flow model to determine the estimated fair value of its investment in ARS as of December 31, 2017 . The assumptions used in preparing the discounted cash flow model include estimates for interest rates, credit spreads, timing and amount of contractual cash flows, liquidity risk premiums, expected holding periods and default risk. The Company updates the discounted cash flow model on a quarterly basis to reflect any changes in the assumptions used in the model and settlements of ARS investments that occurred during the period. The only significant unobservable input in the discounted cash flow model is the discount rate. The discount rate used represents the Company's estimate of the yield expected by a market participant from the ARS investments. The weighted average discount rate used in the discounted cash flow model as of December 31, 2017 and 2016 was approximately 6% and 5% , respectively. Selecting another discount rate within the range used in the discounted cash flow model would not result in a significant change to the fair value of the ARS. Based on this assessment of fair value, as of December 31, 2017 , the Company determined there was a decline in the fair value of its ARS investments of approximately $730,000 . The decline was deemed to be a temporary impairment and recorded as an unrealized loss in accumulated other comprehensive loss in stockholders’ equity. In addition, while a majority of the ARS are currently rated AAA, if the issuers are unable to successfully close future auctions and/or their credit ratings deteriorate, the Company may be required to record additional unrealized losses in accumulated other comprehensive loss or an other-than-temporary impairment charge to earnings on these investments.

PROPERTY AND EQUIPMENT

PROPERTY AND EQUIPMENT	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
PROPERTY AND EQUIPMENT	PROPERTY AND EQUIPMENT Property and equipment consists of the following (in thousands): December 31, 2017 2016 Leasehold improvements $ 59,447 $ 53,073 Furniture, office equipment and vehicles 52,163 45,035 Computer hardware and software 71,281 64,577 Property and equipment, gross 182,891 162,685 Accumulated depreciation and amortization (98,395 ) (75,117 ) Property and equipment, net $ 84,496 $ 87,568 Depreciation expense for property and equipment was approximately $26 million , $24 million and $20 million , for the years ended December 31, 2017 , 2016 , and 2015 , respectively.

GOODWILL

GOODWILL	12 Months Ended
	Dec. 31, 2017
Goodwill [Abstract]
GOODWILL	GOODWILL The changes in the carrying amount of goodwill by operating segment consist of the following (in thousands): North America International Total Goodwill, December 31, 2015 $ 1,227,310 $ 25,635 $ 1,252,945 Acquisition 467 5,933 6,400 Effect of foreign currency translation — (4,479 ) (4,479 ) Goodwill, December 31, 2016 1,227,777 27,089 1,254,866 Acquisitions 25,717 — 25,717 Effect of foreign currency translation — 2,874 2,874 Goodwill, December 31, 2017 $ 1,253,494 $ 29,963 $ 1,283,457 The Company recorded goodwill of approximately $6 million in connection with the May 3, 2016 acquisition of Thomas Daily GmbH (“Thomas Daily”), a commercial real estate news and information provider operating in Freiburg, Germany. Additionally, the Company recorded goodwill of approximately $0.5 million during the year ended December 31, 2016 in connection with the acquisition of certain assets related to the business operations of Apartment Finder's independent distributors within various markets. The Company recorded goodwill of approximately $8 million in connection with the January 31, 2017 acquisition of Koa Lei, Inc. (doing business as Westside Rentals ® and now known as Westside Rentals, LLC), an online marketplace specializing in Southern California real estate rentals, and its affiliated entity Westside Credit Services, LLC, a provider of credit checks and tenant screening for landlords in the Southern California real estate rental market. The Company recorded goodwill of approximately $15 million in connection with the May 10, 2017 acquisition of certain assets and assumption of certain liabilities from Datasphere Technologies, Inc., in each case, related to the LandWatch.com ® business (collectively referred to as “LandWatch”), a leading listing site dedicated to land and rural properties. The Company recorded goodwill of approximately $2 million in connection with the July 18, 2017 acquisition of The Screening Pros, LLC, an online apartment leasing platform that includes tenant screening services, rental applications and payments processing and lease renewals. The purchase accounting for the acquisitions of Westside Rentals®, LandWatch®, and The Screening Pros TM is preliminary, subject to the completion of the accounting for certain tax related items and working capital adjustments. The total amount of goodwill that is expected to be deductible for tax purposes is approximately $24 million as of December 31, 2017 . No goodwill was deductible as of December 31, 2016 . During the fourth quarters of 2017 , 2016 and 2015 , the Company completed the annual impairment test of goodwill and concluded that goodwill was not impaired.

INTANGIBLE ASSETS

INTANGIBLE ASSETS	12 Months Ended
	Dec. 31, 2017
Intangible Assets, Net (Excluding Goodwill) [Abstract]
INTANGIBLE ASSETS	INTANGIBLE ASSETS Intangible assets consist of the following (in thousands, except amortization period data): December 31, Weighted- Average Amortization Period (in years) 2017 2016 Capitalized product development cost $ 2,275 $ 2,275 4 Accumulated amortization (2,262 ) (2,217 ) Capitalized product development cost, net 13 58 Building photography 18,739 17,271 4 Accumulated amortization (18,212 ) (16,351 ) Building photography, net 527 920 Acquired database technology 83,469 78,151 5 Accumulated amortization (79,188 ) (72,691 ) Acquired database technology, net 4,281 5,460 Acquired customer base 225,879 220,749 10 Accumulated amortization (169,157 ) (150,445 ) Acquired customer base, net 56,722 70,304 Acquired trade names and other intangible assets 167,718 153,607 13 Accumulated amortization (46,369 ) (34,384 ) Acquired trade names and other intangible assets, net 121,349 119,223 Intangible assets, net $ 182,892 $ 195,965 Amortization expense for intangible assets was approximately $37 million , $46 million and $59 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. In the aggregate, the Company expects amortization for intangible assets existing as of December 31, 2017 for future periods to be approximately $29 million , $24 million , $21 million , $20 million and $17 million for the years ending December 31, 2018, 2019, 2020, 2021 and 2022, respectively. Intangible assets are reviewed for impairment at least annually and more frequently whenever events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. During the first quarter of 2016, the Company determined that the acquired trade names recorded in connection with the LoopNet acquisition on April 30, 2012 should be reclassified from an indefinite-lived intangible asset to a definite-lived intangible asset due to work being performed to integrate the backend systems of LoopNet and CoStar, which may result in a future re-branding effort if aspects of the LoopNet and CoStar services are ultimately combined. The Company estimated the fair value of the LoopNet trade names using the relief from royalty method and concluded that no impairment existed as of March 31, 2016. The Company estimated a useful life of fifteen years for the LoopNet trade names, which are being amortized on a straight-line basis.

LONG-TERM DEBT

LONG-TERM DEBT	12 Months Ended
	Dec. 31, 2017
Long-term Debt, Current and Noncurrent [Abstract]
LONG-TERM DEBT	LONG-TERM DEBT On April 1, 2014 (the “Closing Date”), the Company entered into the 2014 Credit Agreement by and among the Company, as Borrower, CoStar Realty Information, Inc., as Co-Borrower, the Lenders from time to time party thereto and JPMorgan Chase Bank, N.A., as Administrative Agent. The 2014 Credit Agreement provided for a $400 million term loan and a $225 million revolving credit facility, each with a term of five years . The Company repaid the remaining balance of $310 million and accrued interest on its existing term loan under the 2014 Credit Agreement on October 19, 2017 from existing cash balances. The revolving credit facility includes a subfacility for swingline loans of up to $10 million , and up to $10 million of the revolving credit facility is available for the issuance of letters of credit. The Company has an irrevocable standby letter of credit outstanding totaling $0.2 million as of December 31, 2017 and December 31, 2016 , which was required to secure its San Francisco office lease. The letter of credit was established in 2014 related to the San Francisco office lease, and automatically renews through January 31, 2025. The term loan was amortized in quarterly installments in amounts resulting in an annual amortization of 5% during each of the first, second and third years, 10% during the fourth year and 15% during the fifth year after the Closing Date, with the remainder payable at final maturity. The loans under the 2014 Credit Agreement bore interest, at the Company's option, either (i) during any interest period selected by the Company, at the London interbank offered rate for deposits in U.S. dollars with a maturity comparable to such interest period, adjusted for statutory reserves (“LIBOR”), plus an initial spread of 2% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio (as defined in the 2014 Credit Agreement) of the Company, or (ii) at the greatest of (x) the prime rate from time to time announced by JPMorgan Chase Bank, N.A., (y) the federal funds effective rate plus 0.5% and (z) LIBOR for a one-month interest period plus 1% , plus an initial spread of 1% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio of the Company. If an event of default occurred under the 2014 Credit Agreement, the interest rate on overdue amounts increased by 2% per annum. The obligations under the 2014 Credit Agreement were guaranteed by all material subsidiaries of the Company and were secured by a lien on substantially all of the assets of the Company and those of its material subsidiaries, in each case subject to certain exceptions, pursuant to security and guarantee documents entered into on the Closing Date. On October 19, 2017, the Company entered into an amended and restated credit agreement (the ‘‘2017 Credit Agreement’’), which amended and restated in its entirety the existing 2014 Credit Agreement. The 2017 Credit Agreement provides for a $750 million revolving credit facility with a term of five years from a syndicate of financial institutions as lenders and issuing banks. The 2017 facility may be used for working capital and other general corporate purposes of the Company and its subsidiaries. In connection with the transaction, the Company incurred $4 million of issuance costs. Those costs along with the $5 million of unamortized costs related to the prior agreement were allocated between the extinguishment of the 2014 Credit Agreement and the 2017 Credit Agreement. This allocation resulted in the Company recognizing a loss of $4 million on the extinguishment with the remaining $4 million being deferred and amortized on a straight-line basis as interest expense over the term of the 2017 Credit Agreement. Up to $20 million of the revolving credit facility is available for the issuance of letters of credit. The loans under the 2017 Credit Agreement bear interest during any interest period selected by the Company, at either (i) the London interbank offered rate for deposits in U.S. dollars with a maturity comparable to such interest period, adjusted for statutory reserves (“LIBOR”), plus an initial spread of 1.25% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio (as defined in the 2017 Credit Agreement) of the Company, or (ii) at the greatest of (x) the prime rate from time to time announced by JPMorgan Chase Bank, N.A., (y) the federal funds effective rate plus ½ of 1% and (z) LIBOR for a one-month interest period plus 1.00% , plus an initial spread of 0.25% per annum, subject to adjustment based on the First Lien Secured Leverage Ratio of the Company. If an event of default occurs under the 2017 Credit Agreement, the interest rate on overdue amounts will increase by 2.00% per annum. The obligations under the 2017 Credit Agreement are guaranteed by all material subsidiaries of the Company and are secured by a lien on substantially all of the assets of the Company and its material subsidiaries, in each case subject to certain exceptions, pursuant to security and guarantee agreements entered into on the Closing Date. The 2017 Credit Agreement requires the Company to maintain (i) a First Lien Secured Leverage Ratio not exceeding 3.50 to 1.00 and (ii) after the incurrence of additional indebtedness under certain specified exceptions in the 2017 Credit Agreement, a Total Leverage Ratio (as defined in the 2017 Credit Agreement) not exceeding 4.50 to 1.00. The 2017 Credit Agreement also includes other covenants, including covenants that, subject to certain exceptions, restrict the ability of the Company and its subsidiaries to (i) incur additional indebtedness, (ii) create, incur, assume or permit to exist any liens, (iii) enter into mergers, consolidations or similar transactions, (iv) make investments and acquisitions, (v) make certain dispositions of assets, (vi) make dividends, distributions and prepayments of certain indebtedness, and (vii) enter into certain transactions with affiliates. The Company was in compliance with the covenants in the 2017 Credit Agreement as of December 31, 2017 . As of December 31, 2017 and 2016 , no amounts were outstanding under the Company's credit facility. Total interest expense for the term loan and revolving credit facilities was approximately $9 million , $10 million and $9 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. Interest expense included amortized debt issuance costs of approximately $2 million , $3 million , and $3 million for the years ended December 31, 2017 , 2016 , and 2015 . Total interest paid for the term loan and revolving credit facilities was approximately $6 million , $7 million , and $6 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. 8. LONG-TERM DEBT — (CONTINUED) The Company had no outstanding long-term debt at December 31, 2017 as it had not drawn any amounts under its 2017 Credit Agreement. At December 31, 2017 , the Company had $4 million of deferred debt issuance costs included in deposits and other assets. The following table represents the Company's long-term debt at December 31, 2016 (in thousands): December 31, 2016 Term loan $ 345,000 Debt issuance costs, net (6,661 ) Total debt 338,339 Current maturities of long-term debt (35,000 ) Current debt issuance costs, net 3,134 Total long-term debt, less current portion $ 306,473

INCOME TAXES

INCOME TAXES	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The components of the provision for income taxes attributable to operations consist of the following (in thousands): Year Ended December 31, 2017 2016 2015 Current: Federal $ 41,453 $ 32,198 $ 10,295 State 3,518 3,682 1,503 Foreign 295 76 40 Total current 45,266 35,956 11,838 Deferred: Federal (7,917 ) 12,586 (8,382 ) State 4,695 3,014 2,590 Foreign 319 35 — Total deferred (2,903 ) 15,635 (5,792 ) Total provision for income taxes $ 42,363 $ 51,591 $ 6,046 9. INCOME TAXES — (CONTINUED) The components of deferred tax assets and liabilities consist of the following (in thousands): December 31, 2017 2016 Deferred tax assets: Reserve for bad debts $ 1,636 $ 2,437 Accrued compensation 6,706 5,562 Stock compensation 10,568 14,268 Net operating losses 25,899 30,319 Accrued reserve and other 1,393 2,097 Unrealized loss on securities 185 326 Deferred rent 6,533 7,814 Deferred gain on the sale of building 4,741 8,166 Total deferred tax assets, prior to valuation allowance 57,661 70,989 Valuation allowance (13,032 ) (8,557 ) Total deferred tax assets, net of valuation allowance 44,629 62,432 Deferred tax liabilities: Prepaids (1,239 ) (1,753 ) Depreciation (6,229 ) (13,045 ) Intangibles (43,800 ) (58,747 ) Total deferred tax liabilities (51,268 ) (73,545 ) Net deferred tax assets (liabilities) $ (6,639 ) $ (11,113 ) As of December 31, 2017 and 2016 , a valuation allowance has been established for certain deferred tax assets due to the uncertainty of realization. The valuation allowance as of December 31, 2017 and 2016 includes an allowance for unrealized losses on ARS investments, foreign deferred tax assets and state net operating losses and tax credits. The valuation allowance for the deferred tax asset for unrealized losses on ARS has been recorded as an adjustment to accumulated other comprehensive loss. The Company established the valuation allowance because it is more likely than not that a portion of the deferred tax asset for certain items will not be realized based on the weight of available evidence. A valuation allowance was established for the unrealized losses on securities as the Company has not historically generated capital gains, and it is uncertain whether the Company will generate sufficient capital gains in the future to absorb the capital losses. A valuation allowance was established for the foreign deferred tax assets due to the cumulative loss in recent years in those jurisdictions. The Company has not had sufficient taxable income historically to utilize the foreign deferred tax assets, and it is uncertain whether the Company will generate sufficient taxable income in the future to utilize the deferred tax assets. Similarly, the Company has established a valuation allowance for net operating losses and tax credits in certain states where it is uncertain whether the Company will generate sufficient taxable income to utilize the net operating losses and tax credits before they expire. The Company’s change in valuation allowance was an increase of approximately $4 million for the year ended December 31, 2017 and a decrease of approximately $1 million for the year ended December 31, 2016 . The increase for the year ended December 31, 2017 is due to an increase in the valuation allowance for foreign deferred tax assets related to foreign net operating losses of approximately $4 million . The decrease for the year ended December 31, 2016 is due to a decrease in the valuation allowance for foreign deferred tax assets of approximately $1 million . The Company had U.S. income before income taxes of approximately $167 million , $135 million , and $2 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. The Company had a foreign loss before income taxes of approximately $2 million for the year ended December 31, 2017 . The Company had foreign income before income taxes of approximately $2 million and $1 million for the years ended December 31, 2016 and 2015 , respectively. 9. INCOME TAXES — (CONTINUED) The Company’s provision for income taxes resulted in effective tax rates that varied from the statutory federal income tax rate as follows (in thousands): Year Ended December 31, 2017 2016 2015 Expected federal income tax provision at statutory rate $ 57,770 $ 47,832 $ 903 State income taxes, net of federal benefit 4,776 3,638 (678 ) Foreign income taxes, net effect (3,540 ) (31 ) 469 Increase (decrease) in valuation allowance 3,624 (103 ) 1,956 Nondeductible compensation 230 141 574 Nondeductible transaction costs — 103 229 Meals and entertainment 958 712 1,032 Tax rate changes (7,340 ) 283 1,203 Research credits (20,547 ) (920 ) — Excess tax benefit (7,010 ) — — Tax reserves 12,646 (150 ) 71 Other adjustments 796 86 287 Income tax expense $ 42,363 $ 51,591 $ 6,046 The Company’s U.K. subsidiaries with foreign losses are disregarded entities for U.S. income tax purposes. Accordingly, the losses from these disregarded entities are included in the Company’s consolidated federal income tax provision at the statutory rate. Federal income taxes attributable to income from these disregarded entities are reduced by foreign taxes paid by those disregarded entities. The Company paid approximately $41 million , $34 million , and $1 million in income taxes for the years ended December 31, 2017 , 2016 , and 2015 , respectively. The Company recognized an income tax benefit during the year ended December 31, 2017 for research credits of $21 million for tax years December 31, 2013 through December 31, 2017. These research credits relate to eligible activities including the development of new products, product enhancements and new or improved processes. The Company has net operating loss carryforwards for international income tax purposes of approximately $49 million , which do not expire. The Company has federal net operating loss carryforwards of approximately $38 million that begin to expire in 2020 , state net operating loss carryforwards with a tax value of approximately $6 million that begin to expire in 2020 and state income tax credit carryforwards with a tax value of approximately $3 million that begin to expire in 2020 . The Company realized a cash benefit relating to the use of its tax loss carryforwards of approximately $7 million , $5 million , and $1 million in December 31, 2017 , 2016 , and 2015 , respectively. The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands): Unrecognized tax benefits as of December 31, 2014 $ 5,749 Increase for current year tax positions — Increase for prior year tax positions 1,954 Expiration of the statute of limitation for assessment of taxes (39 ) Unrecognized tax benefits as of December 31, 2015 7,664 Increase for current year tax positions 368 Decrease for prior year tax positions (6,115 ) Expiration of the statute of limitation for assessment of taxes (74 ) Unrecognized tax benefits as of December 31, 2016 1,843 Increase for current year tax positions 12,620 Decrease for prior year tax positions (34 ) Expiration of the statute of limitation for assessment of taxes (66 ) Unrecognized tax benefits as of December 31, 2017 $ 14,363 9. INCOME TAXES — (CONTINUED) Approximately $14 million and $1 million of the unrecognized tax benefits as of December 31, 2017 and 2016 , respectively, would favorably affect the annual effective tax rate, if recognized in future periods. The increase for current year tax positions of $13 million for the year ended December 31, 2017 is primarily attributable to research credits. The Company recognized $72,000 for interest and penalties in its consolidated statement of operations for the year ended December 31, 2017. The Company reversed interest and penalties of $416,000 in its consolidated statement of operations for the year ended December 31, 2016 . The Company recognized $83,000 for interest and penalties in its consolidated statements of operations for the year ended December 31, 2015. The Company had liabilities of $205,000 , $133,000 , and $549,000 for interest and penalties in its consolidated balance sheets as of December 31, 2017 , 2016 , 2015 respectively. The Company does not anticipate the amount of the unrecognized tax benefits will change significantly over the next twelve months. The Company is subject to taxation in the U.S. and various states and foreign jurisdictions. The Company’s federal income tax returns for tax years 2011 through 2016 remain open to examination. Most of the Company’s state income tax returns for tax years 2014 through 2016 remain open to examination. For states that have a four-year statute of limitations, the state income tax returns for tax years 2013 through 2016 remain open to examination. The Company’s U.K. income tax returns for tax years 2012 through 2016 remain open to examination. On December 22, 2017, the U.S. enacted the Tax Cuts and Jobs Act (the “Tax Act”), which significantly changed U.S. tax law. The Tax Act lowered the Company’s U.S. statutory federal income tax rate from 35% to 21% effective January 1, 2018, while also imposing a deemed repatriation tax on deferred foreign income. The Tax Act also created a new minimum tax on certain future foreign earnings. The Securities and Exchange Commission staff issued Staff Accounting Bulletin 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act, ("SAB 118"), which provides guidance on accounting for the tax effects of the Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the Tax Act enactment date for companies to complete the accounting under Accounting Standards Codification Topic 740, ("ASC 740"). In accordance with SAB 118, a company must reflect the income tax effects of those aspects of the Tax Act for which the accounting under ASC 740 is complete. To the extent that a company’s accounting for certain income tax effects of the Tax Act is incomplete but it is able to determine a reasonable estimate, it must record a provisional estimate in the financial statements. If a company cannot determine a provisional estimate to be included in the financial statements, it should continue to apply ASC 740 on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the Tax Act. In connection with the Company's initial analysis of the impact of the Tax Act, the Company recorded a provisional discrete net tax benefit of $7 million in the period ending December 31, 2017. This net benefit primarily consists of a net benefit for the corporate tax rate reduction of $7.4 million and a net expense for the repatriation tax of $400,000 . For various reasons, the Company has not completed its accounting for the income tax effects of certain elements of the Tax Act. The Company was able to make reasonable estimates of the effects of elements for which the Company's analysis is not yet complete and recorded provisional adjustments. As the Company completes its analysis of the Tax Act, collects and prepares necessary data, and interprets any additional guidance issued by the U.S. Treasury Department, the IRS, and other standard-setting bodies, the Company may make adjustments to the provisional amounts. Those adjustments may materially impact the provision for income taxes in the period in which the adjustments are made. Global intangible low taxed income (GILTI): Due to the complexity of the new GILTI tax rules, the Company is continuing to evaluate this provision of the Tax Act and the application of ASC 740. Under U.S. GAAP, the Company is allowed to make an accounting policy choice of either (1) treating taxes due on future U.S. inclusions in taxable income related to GILTI as a current-period expense when incurred (the “period cost method”) or (2) factoring such amounts into the measurement of deferred taxes (the “deferred method”). The Company's selection of an accounting policy with respect to the new GILTI tax rules will depend, in part, on analyzing its global income to determine whether it expects to have future U.S. inclusions in taxable income related to GILTI and, if so, what the impact is expected to be. Because whether the Company expects to have future U.S. inclusions in taxable income related to GILTI depends on not only its current structure and estimated future results of global operations but also its intent and ability to modify its structure and/or its business, the Company is not yet able to reasonably estimate the effect of this provision of the Tax Act. Therefore, the Company has not made any adjustments related to potential GILTI tax in its financial statements and has not made a policy decision regarding whether to record deferred taxes on GILTI.

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES The Company leases office facilities and office equipment under various non-cancelable operating leases. The leases contain various renewal options. Rent expense for the years ended December 31, 2017 , 2016 and 2015 , was approximately $26 million , $22 million and $21 million , respectively. Future minimum lease payments as of December 31, 2017 are as follows (in thousands): 2018 $ 30,853 2019 27,925 2020 26,136 2021 24,544 2022 22,883 Thereafter 46,061 Total future minimum lease payments $ 178,402 Currently, and from time to time, the Company is involved in litigation incidental to the conduct of its business. In accordance with GAAP, the Company records a provision for a liability when it is both probable that a liability has been incurred and the amount can be reasonably estimated. At the present time, while it is reasonably possible that an unfavorable outcome may occur as a result of one or more of the Company’s current litigation matters, management has concluded that it is not probable that a loss has been incurred in connection with the Company’s current litigation. In addition, the Company is unable to estimate the possible loss or range of loss that could result from an unfavorable outcome in the Company’s current litigation and accordingly, the Company has not recognized any liability in the consolidated financial statements for unfavorable results, if any. Legal defense costs are expensed as incurred.

SEGMENT REPORTING

SEGMENT REPORTING	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
SEGMENT REPORTING	SEGMENT REPORTING Segment Information The Company manages its business geographically in two operating segments, with the primary areas of measurement and decision-making being North America, which includes the U.S. and Canada, and International, which includes the U.K., Spain, Germany and France. The Company and its subsidiaries' subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. The Company’s subscription-based information services consist primarily of CoStar Suite ® services. CoStar Suite is sold as a platform of service offerings consisting of CoStar Property Professional ® , CoStar COMPS Professional ® and CoStar Tenant ® and through the Company's mobile application, CoStar Go ® . CoStar Suite is the Company’s primary service offering in the North America and International operating segments. Management relies on an internal management reporting process that provides revenue and operating segment net income (loss) before interest and other income (expense), income taxes, depreciation and amortization (“EBITDA”). Management believes that operating segment EBITDA is an appropriate measure for evaluating the operational performance of the Company’s operating segments. EBITDA is used by management to internally measure operating and management performance and to evaluate the performance of the business. However, this measure should be considered in addition to, not as a substitute for or superior to, income from operations or other measures of financial performance prepared in accordance with GAAP. Summarized information by operating segment consists of the following (in thousands): Year Ended December 31, 2017 2016 2015 Revenues North America $ 934,464 $ 809,492 $ 686,573 International External customers 30,766 28,138 25,191 Intersegment revenue 42 40 41 Total International revenue 30,808 28,178 25,232 Intersegment eliminations (42 ) (40 ) (41 ) Total revenues $ 965,230 $ 837,630 $ 711,764 EBITDA North America $ 236,906 $ 210,901 $ 87,092 International 553 4,169 2,895 Total EBITDA $ 237,459 $ 215,070 $ 89,987 The reconciliation of net income (loss) to EBITDA consists of the following (in thousands): Year Ended December 31, 2017 2016 2015 Net income (loss) $ 122,695 $ 85,071 $ (3,465 ) Amortization of acquired intangible assets in cost of revenues 19,707 22,819 30,077 Amortization of acquired intangible assets in operating expenses 17,684 22,731 27,931 Depreciation and other amortization 26,252 24,615 20,524 Interest and other income (4,044 ) (1,773 ) (537 ) Interest and other expense 9,014 10,016 9,411 Loss on debt extinguishment 3,788 — — Income tax expense 42,363 51,591 6,046 EBITDA $ 237,459 $ 215,070 $ 89,987 11. SEGMENT REPORTING — (CONTINUED) Segment Information — (Continued) Intersegment revenues were attributable to services performed for the Company's wholly owned subsidiary, CoStar Portfolio Strategy by Grecam S.A.S. (“Grecam”), a wholly owned subsidiary of CoStar Limited, the Company’s wholly owned U.K. holding company. Intersegment revenues are recorded at an amount the Company believes approximates fair value. North America EBITDA includes a corresponding cost for the services performed by Grecam. Summarized information by operating segment consists of the following (in thousands): December 31, 2017 2016 Property and equipment, net North America $ 79,736 $ 84,727 International 4,760 2,841 Total property and equipment, net $ 84,496 $ 87,568 Goodwill North America $ 1,253,494 $ 1,227,777 International 29,963 27,089 Total goodwill $ 1,283,457 $ 1,254,866 Assets North America $ 2,816,156 $ 2,139,896 International 57,285 45,167 Total assets $ 2,873,441 $ 2,185,063 Liabilities North America $ 201,831 $ 520,833 International 20,360 10,017 Total liabilities $ 222,191 $ 530,850 11. SEGMENT REPORTING — (CONTINUED) Revenues by Services The Company provides information, analytics and online marketplaces to the commercial real estate industry. The revenues by type of service consists of the following (in thousands): Year Ended December 31, 2017 2016 2015 Information and analytics CoStar Suite (1) $ 463,185 $ 408,456 $ 360,440 Information services (2) 72,618 77,178 75,790 Online marketplaces Multifamily (3) 279,855 224,835 160,630 Commercial property and land (4) 149,572 127,161 114,904 Total revenues $ 965,230 $ 837,630 $ 711,764 (1) CoStar Suite is comprised of: CoStar Property Professional, CoStar COMPS Professional, CoStar Tenant; and CoStar Portfolio Strategy. (2) Information services is comprised of: LoopNet Premium Searcher; CoStar Real Estate Manager; CoStar Risk Analytics COMPASS; CoStar Investment Analysis Portfolio Maximizer; CoStar Investment Analysis Request; CoStar Brokerage Applications; PROPEX; Grecam; Belbex; and Thomas Daily. (3) Multifamily is comprised of Apartments.com, ApartmentFinder.com and ApartmentHomeLiving.com; WestsideRentals.com; Apartamentos.com; and The Screening Pros. (4) Commercial property and land is comprised of: LoopNet Premium Lister; LoopLink; CoStar Advertising; BizBuySell and BizQuest; LandsofAmerica,LandAndFarm, and LandWatch; and CoStar Private Sale Network.

STOCKHOLDERS' EQUITY

STOCKHOLDERS' EQUITY	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
STOCKHOLDERS' EQUITY	STOCKHOLDERS’ EQUITY Preferred Stock The Company has 2,000,000 shares of preferred stock, $0.01 par value, authorized for issuance as of December 31, 2017 . The Board of Directors may issue the preferred stock from time to time as shares of one or more classes or series. Common Stock The Company has 60,000,000 shares of common stock, $0.01 par value, authorized for issuance. Dividends may be declared and paid on the common stock, subject in all cases to the rights and preferences of the holders of preferred stock and authorization by the Board of Directors. In the event of liquidation or winding up of the Company and after the payment of all preferential amounts required to be paid to the holders of any series of preferred stock, any remaining funds shall be distributed among the holders of the issued and outstanding common stock. Equity Offering In October 2017, the Company completed a public equity offering of 3,317,308 shares of common stock for $260.00 per share. Net proceeds from the public equity offering were approximately $834 million , after deducting approximately $29 million of underwriting discounts and other fees. The Company expects to use the net proceeds from the public equity offering to fund all or a portion of the costs of any strategic acquisitions we determine to pursue in the future, to finance the growth of its business and for working capital and other general corporate purposes. General corporate purposes may include additions to working capital, capital expenditures, repayment of debt, investments in the Company’s subsidiaries, possible acquisitions and the repurchase, redemption or retirement of securities, including the Company’s common stock.

NET INCOME (LOSS) PER SHARE

NET INCOME (LOSS) PER SHARE	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
NET INCOME (LOSS) PER SHARE	NET INCOME (LOSS) PER SHARE The following table sets forth the calculation of basic and diluted net income (loss) per share (in thousands except per share data): Year Ended December 31, 2017 2016 2015 Numerator: Net income (loss) $ 122,695 $ 85,071 $ (3,465 ) Denominator: Denominator for basic net income (loss) per share — weighted-average outstanding shares 33,200 32,167 31,950 Effect of dilutive securities: Stock options and restricted stock 359 269 — Denominator for diluted net income (loss) per share — weighted-average outstanding shares 33,559 32,436 31,950 Net income (loss) per share — basic $ 3.70 $ 2.64 $ (0.11 ) Net income (loss) per share — diluted $ 3.66 $ 2.62 $ (0.11 ) Stock options to purchase approximately 87,000 and 194,000 shares that were outstanding for the years ended December 31, 2017 and December 31, 2016 , respectively, were not included in the computation of diluted net income per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. The Company did not consider the impact of potentially dilutive securities for the year ended December 31, 2015 when calculating the diluted net loss per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Shares underlying restricted common stock awards that vest based on Company performance and service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. Shares underlying restricted stock units that vest based on Company service conditions, that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. The following table summarizes the shares underlying the performance-based restricted stock awards and service-based restricted stock units excluded from the basic and diluted calculation (in thousands): Year Ended December 31, 2017 2016 2015 Performance-based restricted stock awards 58 59 55 Service-based restricted stock units 1 1 1 Total shares excluded from computation 59 60 56

EMPLOYEE BENEFIT PLANS

EMPLOYEE BENEFIT PLANS	12 Months Ended
	Dec. 31, 2017
Compensation Related Costs [Abstract]
EMPLOYEE BENEFIT PLANS	EMPLOYEE BENEFIT PLANS Stock Incentive Plans In April 2007, the Company’s Board of Directors adopted the CoStar Group, Inc. 2007 Stock Incentive Plan (as amended, the “2007 Plan”), subject to stockholder approval, which was obtained on June 7, 2007. In April 2016, the Company’s Board of Directors adopted the CoStar Group, Inc. 2016 Stock Incentive Plan (as amended, the “2016 Plan”), subject to stockholder approval, which was obtained on June 9, 2016. All shares of common stock that were authorized for issuance under the 2007 Plan that, as of June 9, 2016, remained available for issuance under the 2007 Plan (excluding shares subject to outstanding awards) were rolled into the 2016 Plan and, as of that date, no shares of common stock were available for new awards under the 2007 Plan. The 2007 Plan continues to govern unexercised and unexpired awards issued under the 2007 Plan prior to June 9, 2016. The 2007 Plan provided for the grant of stock options, restricted stock, restricted stock units and stock appreciation rights to officers, directors and employees of the Company and its subsidiaries. Stock options granted under the 2007 Plan could be incentive or non-qualified, and except in limited circumstances related to a merger or other acquisition, the exercise price for a stock option may not be less than the fair market value of the Company’s common stock on the date of grant. The vesting period of the options, restricted stock and restricted stock unit grants under the 2007 Plan was determined by the Board of Directors or a committee thereof and was generally three to four years . In some cases, vesting of restricted stock awards under the 2007 Plan is subject to performance conditions. Upon the occurrence of a Change of Control, as defined in the 2007 Plan, all outstanding unexercisable options and restricted stock grants under the 2007 Plan immediately become exercisable. The 2016 Plan provides for the grant of stock options, restricted stock, restricted stock units, and stock appreciation rights to officers, directors and employees of the Company and its subsidiaries. Stock options granted under the 2016 Plan may be non-qualified or may qualify as incentive stock options. Except in limited circumstances related to a merger or other acquisition, the exercise price for an option may not be less than the fair market value of the Company’s common stock on the date of grant. The vesting period for each grant of options, restricted stock, restricted stock units and stock appreciation rights under the 2016 Plan is determined by the Board of Directors or a committee thereof and is generally three to four years , subject to minimum vesting periods for restricted stock and restricted stock units of at least one year . In some cases, vesting of awards under the 2016 Plan may be based on performance conditions. The Company has issued and/or reserved the following shares of common stock for issuance under the 2016 Plan: (a) 1,450,000 shares of common stock, plus (b) 815,464 shares of common stock that were authorized for issuance under the 2007 Plan that, as of June 9, 2016, remained available for issuance under the 2007 Plan (not including any Shares that were subject as of such date to outstanding awards under the 2007 Plan), and (c) any shares of common stock subject to outstanding awards under the 2007 Plan as of June 9, 2016, that on or after such date cease for any reason to be subject to such awards (other than by reason of exercise or settlement of the awards to the extent they are exercised for or settled in vested and nonforfeitable shares). Unless terminated sooner, the 2016 Plan will terminate in June 2026, but will continue to govern unexercised and unexpired awards issued under the 2016 Plan prior to that date. Approximately 2 million shares were available for future grant under the 2016 Plan as of December 31, 2017 . At December 31, 2017 , there was approximately $57 million of unrecognized compensation cost related to stock incentive plans, net of estimated forfeitures, which the Company expects to recognize over a weighted-average-period of 2.2 years . 14. EMPLOYEE BENEFIT PLANS — (CONTINUED) Stock Options Option activity was as follows: Number of Shares Range of Exercise Price Weighted- Average Exercise Price Weighted- Average Remaining Contract Life (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2014 370,179 $36.48 - $201.04 $ 99.12 Granted 89,500 $193.69 - $193.69 $ 193.69 Exercised (59,602 ) $36.48 - $201.04 $ 85.48 Outstanding at December 31, 2015 400,077 $36.48 - $201.04 $ 122.30 Granted 82,400 $182.75 - $182.75 $ 182.75 Exercised (29,285 ) $36.48 - $201.04 $ 112.78 Canceled or expired (13,034 ) $193.69 - $201.04 $ 195.78 Outstanding at December 31, 2016 440,158 $36.48 - $201.04 $ 132.08 Granted 95,500 $204.91 $ 204.91 Exercised (81,815 ) $36.48 - $201.04 $ 83.07 Outstanding at December 31, 2017 453,843 $36.73 - $204.91 $ 156.24 6.67 $ 63,861 Exercisable at December 31, 2015 220,107 $36.48 - $201.04 $ 77.63 Exercisable at December 31, 2016 284,489 $36.48 - $201.04 $ 100.94 Exercisable at December 31, 2017 278,239 $36.73 - $201.04 $ 130.91 5.47 $ 46,199 The aggregate intrinsic value is calculated as the difference between (i) the closing price of the common stock at the end of the period and (ii) the exercise prices of the underlying awards, multiplied by the shares underlying options as of the end of the period that had an exercise price less than the closing price on that date. Options to purchase 81,815 , 29,285 , and 59,602 , shares were exercised during the years ended 2017 , 2016 , and 2015 , respectively. The aggregate intrinsic value of options exercised, determined as of the date of option exercise, was approximately $13 million , $3 million , and $7 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. The weighted-average grant date fair value of each option granted during the years ended December 31, 2017 , 2016 , and 2015 using the Black-Scholes option-pricing model was $59.06 , $54.34 and $56.53 , respectively. The Company estimated the fair value of each option granted on the date of grant using the Black-Scholes option-pricing model, using the assumptions in the following table: Year Ended December 31, 2017 2016 2015 Dividend yield 0 % 0 % 0 % Expected volatility 28 % 31 % 30 % Risk-free interest rate 2 % 1 % 2 % Expected life (in years) 5 5 5 14. EMPLOYEE BENEFIT PLANS — (CONTINUED) Stock Options — (Continued) The expected dividend yield is determined based on the Company's past cash dividend history and anticipated future cash dividend payments. The Company has never declared or paid any dividends on its common stock and does not anticipate paying any dividends on its common stock during the foreseeable future, but intends to retain any earnings for future growth of its business. Expected volatility is calculated based on historical volatility of the daily closing price of the Company's common stock over a period consistent with the expected life of the options granted. The risk-free interest rate is based on the U.S. Treasury rate with terms similar to the expected life of the options granted. The expected life for the options is determined based on multiple factors, including historical employee behavior patterns of exercising options and post-employment termination behavior as well as expected future employee option exercise patterns. The following table summarizes information regarding options outstanding at December 31, 2017 : Options Outstanding Options Exercisable Range of Exercise Price Number of Shares Weighted-Average Remaining Contractual Life (in years) Weighted- Average Exercise Price Number of Shares Weighted- Average Exercise Price $36.73 - $58.06 41,359 3.13 $ 56.64 41,359 $ 56.64 $58.07 - $59.59 40,004 4.14 $ 58.95 40,004 $ 58.95 $59.60 - $81.19 1,000 3.42 $ 60.23 1,000 $ 60.23 $81.20 - $142.45 70,944 5.19 $ 102.16 70,944 $ 102.16 $142.46 - $188.22 76,934 8.19 $ 182.75 21,999 $ 182.75 $188.23 - $197.37 65,802 7.17 $ 193.69 40,633 $ 193.69 $197.38 - $202.98 62,300 6.16 $ 201.04 62,300 $ 201.04 $202.99 - $204.91 95,500 9.16 $ 204.91 — $ — $36.73 - $204.91 453,843 6.67 $ 156.24 278,239 $ 130.91 Restricted Stock Awards In March 2017, March 2016, and March 2015, the Compensation Committee of the Board of Directors of the Company approved grants of restricted common stock to the executive officers that vest based on the Company’s achievement of a three-year cumulative revenue goal established at the grant date, and are subject to forfeiture in the event the foregoing performance condition is not met by the end of each respective three-year period. The number of shares that may be earned ranges between 0% (if the specified threshold performance level is not attained) and 200% (if performance meets or exceeds the maximum achievement level) of the target award. If actual performance exceeds the pre-established threshold, the number of shares earned is calculated based on the relative performance between specified levels of achievement. These awards support the Company’s goals of aligning executive incentives with long-term stockholder value and ensuring that executive officers have a continuing stake in the long-term success of the Company. These grants of restricted common stock are subject to continuing employment requirements and to a market condition. The actual number of shares that vest at the end of the respective three-year period is determined based on the Company’s achievement of the three -year performance goals described above, as well as its TSR relative to the Russell 1000 Index over the same three-year performance period. At the end of the three-year performance period, if the performance condition is achieved at or above the pre-established threshold, the number of shares earned is further adjusted by a TSR payout percentage, which ranges between 80% and 120% , based on the Company’s TSR performance relative to that of the Russell 1000 Index over the respective three-year period. The Company granted a total of 32,160 , 25,680 and 32,400 shares of performance-based restricted common stock during the years ended December 31, 2017 , 2016 , and 2015 , respectively. 14. EMPLOYEE BENEFIT PLANS — (CONTINUED) Restricted Stock Awards — (Continued) The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures. The assumptions used to estimate the fair value of performance-based restricted common stock awards with a market condition granted were as follows: Year Ended December 31, 2017 2016 2015 Dividend yield 0 % 0 % 0 % Expected volatility 28 % 28 % 26 % Risk-free interest rate 2 % 1 % 1 % Expected life (in years) 3 3 3 Weighted-average grant date fair value $ 218.59 $ 184.97 $ 208.08 The expected dividend yield is determined based on the Company's past cash dividend history and anticipated future cash dividend payments. The Company has never declared or paid any dividends on its common stock and does not anticipate paying any dividends on its common stock during the foreseeable future, but intends to retain any earnings for future growth of its business. Expected volatility is calculated based on historical volatility of the daily closing price of the common stock of the companies within the Russell 1000 Index over a period consistent with the expected life of the performance-based restricted common stock awards with a market condition. The risk-free interest rate is based on the U.S. Treasury rate with terms similar to the expected life of the performance-based restricted common stock awards with a market condition. The expected life is consistent with the performance measurement period of the performance-based restricted common stock awards with a market condition. As of December 31, 2017 , the Company reassessed the probability of achieving the performance and market conditions and determined that it was probable that the performance and market conditions for the 2017, 2016 and 2015 performance-based restricted common stock awards would be met by their forfeiture dates. As a result, the Company recorded a total of approximately $5 million , $3 million , and $3 million of stock-based compensation expense related to the performance-based restricted common stock awards with a market condition for the years ended December 31, 2017 , 2016 and 2015 , respectively. The Company expects to record an aggregate amount of stock-based compensation expense related to the performance-based restricted common stock awards of approximately $6 million over the periods 2018, 2019 and 2020 . The following table presents unvested restricted stock awards activity for the year ended December 31, 2017 : Restricted Stock Awards — without Market Condition Restricted Stock Awards — with Market Condition Number of Shares Weighted-Average Grant Date Fair Value per Share Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock awards at December 31, 2016 394,712 $ 192.38 72,480 $ 202.07 Granted 156,238 $ 211.69 32,160 $ 218.59 Vested (156,240 ) $ 177.72 (13,477 ) $ 216.20 Canceled (30,827 ) $ 196.87 (5,963 ) $ 216.20 Unvested restricted stock awards at December 31, 2017 363,883 $ 206.59 85,200 $ 205.08 14. EMPLOYEE BENEFIT PLANS — (CONTINUED) Restricted Stock Units The following table presents unvested restricted stock units activity for the year ended December 31, 2017 : Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock units at December 31, 2016 1,199 $ 182.64 Granted 69 $ 291.82 Vested (306 ) $ 183.28 Canceled — $ — Unvested restricted stock units at December 31, 2017 962 $ 190.27 Employee 401(k) Plan The Company maintains a 401(k) Plan (the “401(k)”) as a defined contribution retirement plan for all eligible employees. The 401(k) provides for tax-deferred contributions of employees’ salaries, limited to a maximum annual amount as established by the IRS. In addition to the traditional 401(k), effective January 1, 2015, eligible employees have the option of making an after-tax contribution to a Roth 401(k) plan or a combination of both. In 2017 , 2016 and 2015 , the Company matched 100% of employee contributions up to a maximum of 4% of total compensation. Amounts contributed to the 401(k) by the Company to match employee contributions for the years ended December 31, 2017 , 2016 , and 2015 were approximately $10 million , $9 million and $8 million , respectively. The Company had no administrative expenses in connection with the 401(k) plan for the years ended December 31, 2017 , 2016 , and 2015 , respectively. Employee Pension Plan The Company maintains a Group Personal Pension Plan (the “Plan”) for all eligible employees in the Company’s U.K. offices. The Plan is a defined contribution plan. Employees are eligible to contribute a portion of their salaries, subject to a maximum annual amount as established by Her Majesty's Revenue and Customs. In 2017 , 2016 and 2015 , the Company's matching contribution was based on the percentage contributed by the employee, up to a maximum of 6% of total compensation. Amounts contributed to the Plan by the Company to match employee contributions for the years ended December 31, 2017 , 2016 , and 2015 , were approximately $380,000 , $380,000 and $420,000 respectively. Registered Retirement Savings Plan As of January 1, 2015, the Company introduced a registered retirement savings plan (“RRSP”) for all eligible employees in the Company’s Canadian offices. In 2017, 2016, and 2015, the Company matched 100% of employee contributions up to a maximum of 4% of total compensation. Amounts contributed to the RRSP by the Company to match employee contributions for the years ended December 31, 2017 , 2016 , and 2015 were approximately $ 43,000 , $10,000 , and $40,000 respectively. Employee Stock Purchase Plan As of August 1, 2006, the Company introduced an Employee Stock Purchase Plan (“ESPP”), pursuant to which eligible employees participating in the plan authorize the Company to withhold specified amounts from the employees’ compensation and use the withheld amounts to purchase shares of the Company's common stock at 90% of the market price. Participating employees are able to purchase common stock under this plan during each offering period. An offering period begins the second Saturday before each of the Company’s regular pay dates and ends on each of the Company’s regular pay dates. On June 3, 2015, the Company’s stockholders approved an amendment to the ESPP to increase the number of shares available for purchase under the ESPP by 100,000 shares. On September 14, 2015, the Company registered the issuance of these additional shares under the ESPP pursuant to the registration statement filed September 14, 2015. There were 80,022 and 93,812 shares available for purchase under the ESPP as of December 31, 2017 and 2016 , respectively, and approximately 13,790 and 14,735 shares of the Company’s common stock were purchased under the ESPP during 2017 and 2016 , respectively.

QUARTERLY RESULTS

QUARTERLY RESULTS	12 Months Ended
	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
QUARTERLY RESULTS	QUARTERLY RESULTS OF OPERATIONS The following is a summary of the unaudited quarterly results of operations for the years ended December 31, 2017 and 2016 . 2017 Mar. 31 Jun. 30 Sep. 30 Dec. 31 Revenues $ 226,553 $ 237,153 $ 247,533 $ 253,991 Cost of revenues 51,346 55,273 55,483 58,301 Gross profit 175,207 181,880 192,050 195,690 Operating expenses 137,545 153,997 134,537 144,932 Income from operations 37,662 27,883 57,513 50,758 Interest and other income 429 605 555 2,455 Interest and other expense (2,686 ) (2,693 ) (2,901 ) (734 ) Loss on debt extinguishment — — — (3,788 ) Income before income taxes 35,405 25,795 55,167 48,691 Income tax expense 13,275 3,611 20,990 4,487 Net income $ 22,130 $ 22,184 $ 34,177 $ 44,204 Net income per share — basic $ 0.69 $ 0.68 $ 1.05 $ 1.24 Net income per share — diluted $ 0.68 $ 0.68 $ 1.04 $ 1.22 2016 Mar. 31 Jun. 30 Sep. 30 Dec. 31 Revenues $ 199,739 $ 206,869 $ 212,711 $ 218,311 Cost of revenues 42,900 42,679 42,222 46,013 Gross profit 156,839 164,190 170,489 172,298 Operating expenses 126,538 136,071 130,893 125,409 Income from operations 30,301 28,119 39,596 46,889 Interest and other income 84 159 344 1,186 Interest and other expense (2,509 ) (2,455 ) (2,498 ) (2,554 ) Income before income taxes 27,876 25,823 37,442 45,521 Income tax expense 11,155 10,247 14,241 15,948 Net income $ 16,721 $ 15,576 $ 23,201 $ 29,573 Net income per share — basic $ 0.52 $ 0.48 $ 0.72 $ 0.92 Net income per share — diluted $ 0.52 $ 0.48 $ 0.72 $ 0.91

SUBSEQUENT EVENTS

SUBSEQUENT EVENTS	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
SUBSEQUENT EVENTS	6. SUBSEQUENT EVENTS On February 21, 2018, the Company completed the acquisition of ForRent, a division of Dominion Enterprises, pursuant to which the Company acquired all of the issued and outstanding shares of common stock, no par value per share, of DE Holdings, Inc., for a purchase price of approximately $385 million , payable approximately $350 million in cash and approximately $35 million in shares of Company common stock, subject to a customary working capital adjustment and other post-closing adjustments.

Schedule II Valuation and Quali

Schedule II Valuation and Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
Valuation and Qualifying Accounts [Abstract]
Schedule II - Valuation and Qualifying Accounts	Schedule II – Valuation and Qualifying Accounts Years Ended December 31, 2017 , 2016 , and 2015 (in thousands): Allowance for Doubtful Accounts and Billing Adjustments (1) Balance at Beginning of Year Charged to Expense Charged to Other Accounts (2) Write-offs, Net of Recoveries Balance at End of Year Year ended December 31, 2015 $ 4,815 $ 7,002 $ 1,470 $ 5,809 $ 7,478 Year ended December 31, 2016 $ 7,478 $ 7,358 $ — $ 8,492 $ 6,344 Year ended December 31, 2017 $ 6,344 $ 5,690 $ — $ 5,565 $ 6,469 (1) Additions to the allowance for doubtful accounts are charged to bad debt expense. (2) Amounts represent opening balances from acquired businesses.

SUMMARY OF SIGNIFICANT ACCOUN26

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Accounting policies are consistent for each operating segment.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. On an ongoing basis, the Company evaluates its estimates and assumptions, including those related to revenue recognition, allowance for doubtful accounts, useful lives of property and equipment and intangible assets, recoverability of long-lived assets and intangible assets with definite lives, goodwill, income taxes, fair value of equity instruments, fair value of auction rate securities (“ARS”), accounting for business combinations and contingencies, among others. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that it believes are reasonable, including assumptions as to future events. These estimates form the basis for making judgments about the carrying values of assets and liabilities and recorded revenues and expenses. Actual results could differ from these estimates.
Revenue Recognition	Revenue Recognition The Company derives revenues by providing access to its proprietary database of commercial real estate information, typically through a fixed monthly fee for its subscription-based services. The Company's subscription-based services consist primarily of information, analytics and online marketplace services offered over the Internet to commercial real estate industry and related professionals. Subscription contract rates are based on the number of sites, number of users, organization size, the client’s business focus, geography, the number and types of services to which a client subscribes, the number of properties a client advertises and the prominence and placement of a client's advertised properties in the search results. A majority of the subscription-based license agreements have a term of one year and renew automatically. Revenues are recognized when (1) there is persuasive evidence of an arrangement, (2) the fee is fixed or determinable, (3) services have been rendered and payment has been contractually earned and (4) collectability is reasonably assured. Revenues from subscription-based services are recognized on a straight-line basis over the term of the agreement. Deferred revenue results from advance cash receipts from customers or amounts billed in advance to customers from the sale of subscription licenses and is recognized over the term of the license agreement. 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) Revenue Recognition — (Continued) The Company analyzes contracts with multiple elements under the accounting guidance for multiple-element arrangements. The Company's multiple-element arrangements include information, analytics and/or online marketplace services that are generally provided to the customer over the same term. When identifying multiple-element arrangements, the Company considers multiple purchases made by the same customer within a short time frame and assesses whether the purchases were negotiated together as one overall arrangement. If a multiple-element arrangement is identified, then the arrangement consideration is allocated among the separate units of accounting based on their relative selling prices, which are estimated considering factors such as historical pricing, pricing strategy, market conditions and other factors. The Company accounts for each deliverable in the transaction separately. If the deliverables cannot be separated into multiple units of accounting, then the arrangement consideration is combined and recognition of revenue is determined for the combined unit of accounting. Multiple-element transactions require judgment to determine the selling price or fair value of the different elements. These judgments impact the amount of revenue recognized over the term of the contract, as well as the period in which they are recognized.
Cost of Revenues	Cost of Revenues Cost of revenues principally consists of salaries, benefits, bonuses, and stock-based compensation expenses for the Company’s researchers who collect and analyze the commercial real estate data that is the basis for the Company’s information, analytics and online marketplaces. Additionally, cost of revenues includes the cost of data from third-party data sources, credit card and other transaction fees relating to processing customer transactions, which are expensed as incurred, and the amortization of acquired trade names and other intangible assets and database technology.
Foreign Currency Translation	Foreign Currency Translation The Company’s functional currency in its foreign locations is the local currency. Assets and liabilities are translated into U.S. dollars using the exchange rates as of the balance sheet dates. Revenues, expenses, gains and losses are translated at the average exchange rates in effect during each period. Gains and losses resulting from translation are included in accumulated other comprehensive loss. Currency g ains and losses on the translation of intercompany loans made to foreign subsidiaries that are of a long-term investment nature are also included in accumulated other comprehensive loss. Net gains or losses resulting from foreign currency exchange transactions are included in the consolidated statements of operations.
Advertising Costs	Advertising Costs The Company expenses advertising costs as incurred. Advertising costs include e-commerce, television, radio, print and other media advertising.
Income Taxes	Income Taxes Deferred income taxes result from temporary differences between the tax basis of assets and liabilities and the basis reported in the Company’s consolidated financial statements. Deferred tax liabilities and assets are determined based on the difference between the financial statement and the tax basis of assets and liabilities using enacted rates in effect during the year in which the Company expects differences to reverse. Valuation allowances are provided against assets, including net operating losses, if the Company anticipates that some or all of an asset may not be realized through future taxable earnings or implementation of tax planning strategies. Interest and penalties related to income tax matters are recognized in income tax expense.
Net Income (Loss) Per Share	Net Income (Loss) Per Share Net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period on a basic and diluted basis. The Company’s potentially dilutive securities include stock options and restricted stock. Diluted net income (loss) per share considers the impact of potentially dilutive securities except in periods in which there is a net loss, as the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Stock options to purchase approximately 87,000 and 194,000 shares that were outstanding for the years ended December 31, 2017 and December 31, 2016 , respectively, were not included in the computation of diluted net income per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. The Company did not consider the impact of potentially dilutive securities for the year ended December 31, 2015 when calculating the diluted net loss per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Shares underlying restricted common stock awards that vest based on Company performance and service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. Shares underlying restricted stock units that vest based on Company service conditions, that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share.
Stock-Based Compensation	Stock-Based Compensation Equity instruments issued in exchange for services performed by officers, employees, and directors of the Company are accounted for using a fair-value based method and the fair value of such equity instruments is recognized as expense in the consolidated statements of operations. Stock-based compensation expense is measured at the grant date of the stock-based awards that vest over set time periods based on their fair values, and is recognized on a straight-line basis as expense over the vesting periods of the awards, net of an estimated forfeiture rate. For equity instruments that vest based on performance, the Company assesses the probability of the achievement of the performance conditions at the end of each reporting period, or more frequently based upon the occurrence of events that may change the probability of whether the performance conditions would be met. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing of recognition may fluctuate from period to period based on those estimates. For equity instruments that vest based on a performance condition and a market condition, the Company estimates the fair value of each equity instrument granted on the date of grant using a Monte-Carlo simulation model. This pricing model uses multiple simulations to evaluate the probability of achieving the market condition to calculate the fair value of the awards. Stock-based compensation expense is updated based on the expected achievement of the related performance conditions at the end of each reporting period. If the performance conditions are not met, no stock-based compensation expense will be recognized, and any previously recognized stock-based compensation expense will be reversed. The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents . Cash equivalents consist of money markets and commercial paper.
Investments	Investments The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities. Investments are carried at fair value. Changes in unrealized holding gains and losses, net of the related tax effect, on available-for-sale securities are excluded from earnings and are reported as a separate component of accumulated other comprehensive loss in stockholders’ equity until realized. A decline in market value of any available-for-sale security below cost that is deemed to be other-than-temporary results in a reduction in carrying amount to fair value. The impairment is charged to earnings and a new cost basis for the security is established. Dividend and interest income are recognized when earned. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis. The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as ARS. Investments are carried at fair value.
Concentration of Credit Risk and Financial Instruments	Concentration of Credit Risk and Financial Instruments The Company performs ongoing assessments of its customers’ financial conditions and generally does not require that its customers’ obligations to the Company be secured. The Company maintains reserves for estimated inherent credit losses, and such losses have been within management’s expectations. The large size and widespread nature of the Company’s customer base and the Company’s lack of dependence on any individual customer mitigates the risk of nonpayment of the Company’s accounts receivable. No single customer accounted for more than 5% of the Company’s revenues for each of the years ended December 31, 2017, 2016, and 2015. The carrying amount of the accounts receivable approximates the net realizable value. The Company holds cash at major financial institutions that often exceed Federal Deposit Insurance Corporation insured limits. The Company manages its credit risk associated with cash concentrations by concentrating its cash deposits in high quality financial institutions and by periodically evaluating the credit quality of the primary financial institutions holding such deposits. The carrying value of cash approximates fair value. Historically, the Company has not experienced any losses due to such cash concentrations.
Accounts Receivable, Net of Allowance for Doubtful Accounts	Accounts Receivable, Net of Allowance for Doubtful Accounts Accounts receivable are recorded at the invoiced amount net of credits due. Accounts receivable payment terms vary and amounts due from customers are stated in the financial statements net of an allowance for doubtful accounts. When evaluating the adequacy of the allowance for doubtful accounts, the Company analyzes historical collection experience, changes in customer payment profiles and the aging of receivable balances, as well as current economic conditions, all of which may affect a customer’s ability to pay.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets: Leasehold improvements Shorter of lease term or useful life Furniture and office equipment Five to ten years Vehicles Five to ten years Computer hardware and software Two to five years Qualifying internal-use software costs incurred during the application development stage, which consist primarily of internal product development costs, outside services and purchased software license costs are capitalized and amortized over the estimated useful life of the asset. All other costs are expensed as incurred.
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill represents the excess of costs over the fair value of assets of acquired businesses. Goodwill is not amortized, but instead tested for impairment at least annually by each reporting unit. The Company may first assess qualitative factors to evaluate whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount or elect to bypass such assessment. If it is determined that it is more likely than not that the fair value of a reporting unit is less than its carrying value, or the Company elects to bypass such assessment, the Company then determines the fair value of each reporting unit. The estimate of the fair value of each reporting unit is based on a projected discounted cash flow model that includes significant assumptions and estimates including the Company's discount rate, growth rate and future financial performance. Assumptions about the discount rate are based on a weighted average cost of capital for comparable companies. Assumptions about the growth rate and future financial performance of a reporting unit are based on the Company's forecasts, business plans, economic projections and anticipated future cash flows. The fair value of each reporting unit is compared to the carrying amount of the reporting unit. If the carrying value of the reporting unit exceeds the fair value, then an impairment loss is recognized for the difference. Acquired database technology, customer base and trade names and other intangible assets are related to the Company’s acquisitions (see Notes 6 and 7 ). Acquired database technology is amortized on a straight-line basis over periods ranging from three years to eight years . Acquired trade names and other intangible assets are amortized on a straight-line basis over periods ranging from three years to fifteen years . See Note 7 for further details on the reclassification of the acquired trade names recorded in connection with the LoopNet acquisition from an indefinite-lived intangible asset to a definite-lived intangible asset. Acquired intangible assets characterized as customer base consists of acquired customer contracts and the related customer relationships and are amortized over periods ranging from ten years to thirteen years . Acquired customer bases are typically amortized on an accelerated basis related to the expected economic benefit of the intangible asset. The cost of capitalized building photography is amortized on a straight-line basis over periods ranging from three years to five years .
Long-Lived Assets	Long-Lived Assets Long-lived assets, such as property and equipment, and purchased intangibles subject to amortization, are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset to estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount of an asset exceeds its estimated future cash flows, an impairment charge is recognized in the amount by which the carrying amount of the asset exceeds the fair value of the asset. Assets to be disposed of would be separately presented in the balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and would no longer be depreciated. The assets and liabilities of a disposal group classified as held for sale would be presented separately in the appropriate asset and liability sections of the balance sheet.
Debt Issuance Costs	Debt Issuance Costs Costs incurred in connection with the issuance of long-term debt are deferred and amortized as interest expense over the term of the related debt using the effective interest method for term debt and on a straight-line basis for revolving debt. These amounts are reflected in the consolidated balance sheets as direct deductions from a combination of the current and long-term portions of debt for term debt and as current and long-term assets for costs related to revolving debt. Upon a refinancing or amendment, previously capitalized debt issuance costs are expensed and included in loss on extinguishment of debt if the Company determines that there has been a substantial modification of the related debt. If the Company determines that there has not been a substantial modification of the related debt, any previously capitalized debt issuance costs are amortized as interest expense over the term of the new debt instrument.
Business Combinations	Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, and acquired trade names from a market participant's perspective, useful lives and discount rates. During the measurement period, the Company may record adjustments to the assets acquired and liabilities assumed. Any adjustments to provisional amounts that are identified during the measurement period are recorded in the reporting period in which the adjustment amounts are determined. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In March 2016, the FASB issued ASU 2016-09, Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting , which simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification of share-based payment transactions on the statement of cash flows. The guidance requires a company to (i) recognize all excess tax benefits and tax deficiencies as income tax expense or benefit in the statement of operations using a prospective transition method, (ii) recognize excess tax benefits in the current period regardless of whether the benefit reduces taxes payable using a modified retrospective transition method, and (iii) classify all excess tax benefits as operating activities within the statement of cash flows using either a prospective transition method or a retrospective transition method. The guidance also allows a company to (i) elect whether to estimate the number of share-based awards expected to vest or account for forfeitures when they occur, and (ii) withhold up to the maximum statutory tax rate in the applicable jurisdiction for awards, both of which should be applied using a modified retrospective transition method. Finally, the guidance requires a company to classify the cash paid by an employer when directly withholding shares for tax withholding purposes as a financing activity within the statement of cash flows using a retrospective transition method. This guidance is effective for annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. The Company adopted the new guidance on January 1, 2017. The adoption of the new standard resulted in a $2 million cumulative-effect adjustment as of January 1, 2017 to record a deferred tax asset with the offset to retained earnings on the balance sheet, representing the amount of the Company's net operating loss carryforwards attributable to excess tax benefits that it was not able to record under the prior guidance. The Company elected to continue to estimate the number of awards expected to be forfeited and adjust the estimate when it is no longer probable that the service or performance conditions will be met. Additionally, the Company elected to apply the presentation requirements for statement of cash flows related to excess tax benefits retrospectively to all periods presented, which resulted in an increase to both net cash provided by operating activities and net cash used in financing activities of approximately $5 million and $9 million for the twelve months ended December 31, 2016 and 2015 , respectively. The presentation requirements related to employee taxes paid by withholding shares had no impact on any of the periods presented in the Company's consolidated cash flows statements since such cash flows have historically been presented as a financing activity. In January 2017, the FASB issued ASU 2017-04, Intangibles - Goodwill and Other (Topic 350: Simplifying the Test for Goodwill Impairment , which is designed to simplify the subsequent measurement of goodwill by eliminating Step 2 from the goodwill impairment test. The guidance indicates that an entity should recognize an impairment charge for the amount by which the carrying amount exceeds the reporting unit's fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to that reporting unit. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2019, including interim periods within that reporting period. Early application is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company early adopted this guidance during the first quarter of 2017 and the early adoption did not have a material impact on the Company's consolidated financial statements and related disclosures. Recent Accounting Pronouncements Not Yet Adopted In May 2014, the Financial Accounting Standards Board (“FASB”) and International Accounting Standards Board (“IASB”) jointly issued a new revenue recognition standard, Accounting Standards Update (“ASU) 2014-09, Revenue from Contracts with Customers (Topic 606) that is designed to improve financial reporting by creating common recognition guidance for GAAP and International Financial Reporting Standards (“IFRS”). This guidance provides a robust framework for addressing revenue issues, improves the comparability of revenue recognition practices across industries, provides useful information to users of financial statements through improved disclosure requirements and simplifies the presentation of financial statements. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goo ds or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. Subsequently, amendments to the new revenue recognition standard were issued to clarify numerous accounting topics, including, but not limited to (i) the implementation guidance on principal versus agent considerations, (ii) the identification of performance obligations, (iii) the licensing implementation guidance, (iv) the objective of the collectability criterion, (v) the application o f the variable consideration guidance and modified retrospective transition method, (vi) the way in which impairment testing is performed and (vii) the disclosure requirements for revenue recognized from performance obligations. This guidance permits the use of either a full retrospective method or a modified retrospective approach. The modified retrospective approach would be applied only to the most current period presented along with a cumulative-effect adjustment at the date of adoption. This guidance will be effective for annual reporting periods beginnin g after December 15, 2017, although companies may adopt the standard as early as annual reporting periods beginning after December 15, 2016, including interim periods within that reporting period. The Company currently intends to adopt the new guidance using the modified retrospective approach. Under this adoption method, the Company will recognize a cumulative effect adjustment to retained earnings as of January 1, 2018 and will account for its contracts with customers under the new guidance prospectively beginning on January 1, 2018. The Company used several available practical expedients provided in the new guidance, including assessing contracts with similar terms and conditions on a “portfolio” basis. Based on the Company’s preliminary analysis, the Company believes that the potential impact of adopting this guidance on reported revenue in any period will not be material. The primary impact of adopting the new guidance relates to the deferral of incremental costs of obtaining customer contracts which is primarily commission costs on new sales. Under existing guidance, the Company expensed all commission costs in the periods they were earned, whereas under the new guidance the Company will defer commission costs on new sales and amortize them on a straight-line basis over three years. The three-year amortization period was determined based on several factors, including the nature of the technology and proprietary data underlying the services being purchased; customer contract renewals rates; and industry competition. The Company currently estimates that the cumulative effect of adoption will result in the recognition of deferred commission cost asset between $48 million to $58 million , net of appropriate taxes, as of January 1, 2018. Beginning in 2018, the Company expects significant changes to its disclosed revenue recognition policies and practices, as well as to other related financial statement disclosures. These revised disclosures will be made in the Company’s first quarterly report in 2018. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842) , to increase transparency and comparability among organizations’ accounting for leases. The guidance requires a company to recognize lease assets and lease liabilities on the balance sheet, as well as disclose key information about leasing arrangements. This guidance is effective on a modified retrospective basis for annual reporting periods beginning after December 15, 2018, including interim periods within that reporting period. Early application is permitted. The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures, but expects that the adoption of this standard may result in a material increase in assets and liabilities on its consolidated balance sheets. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, which is designed to provide financial statement users with more information about the expected credit losses on financial instruments and other commitments to extend credit held by a reporting entity at each reporting date. When determining such expected credit losses, the guidance requires companies to apply a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. This guidance is effective on a modified retrospective basis for annual reporting periods beginning after December 15, 2019, including interim periods within that reporting period. C ompanies may adopt the standard as early as annual reporting periods beginning after December 15, 2018 . The Company is currently evaluating the impact this guidance will have on its financial statements and related disclosures. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments , which is designed to reduce the existing diversity in how certain cash receipts and cash payments are presented and classified in the consolidated statements of cash flows. This guidance is effective on a retrospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. The Company will adopt this guidance in the first quarter of 2018. This guidance is not expected to have a material impact on the Company's consolidated statements of cash flows and related disclosures. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business , which is designed to clarify the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The guidance indicates that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set of assets is not a business. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. The Company will adopt this guidance in the first quarter of 2018 and this guidance is not expected to have a material impact on the Company's consolidated financial statements and related disclosures. 2. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES — (CONTINUED) In May 2017, the FASB issued ASU 2017-09, Compensation- Stock Compensation (Topic 718): Scope of Modification Accounting , which is designed to reduce the existing diversity and complexity in the accounting for changes to terms or conditions of a share-based payment award. This guidance clarifies that an entity will not apply modification accounting to a share-based payment award if all of the following are the same immediately before and after the change: (i) the fair value of the award, (ii) the vesting conditions of the award, and (iii) the classification of the award as an equity instrument or liability instrument. This guidance is effective on a prospective basis for annual reporting periods beginning after December 15, 2017, including interim periods within that reporting period. The Company will adopt this guidance in the first quarter of 2018. This guidance is not expected to have a material impact on the Company's consolidated financial statements and related disclosures.

ACQUISITION (Policies)

ACQUISITION (Policies)	12 Months Ended
	Dec. 31, 2017
Business Acquisition [Line Items]
Acquisitions	Business Combinations The Company allocates the purchase consideration to the tangible assets acquired, liabilities assumed and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is recorded as goodwill. Such valuations require management to make significant estimates and assumptions, especially with respect to intangible assets. Significant estimates in valuing certain intangible assets include, but are not limited to, future expected cash flows from acquired customer bases, acquired database technology, and acquired trade names from a market participant's perspective, useful lives and discount rates. During the measurement period, the Company may record adjustments to the assets acquired and liabilities assumed. Any adjustments to provisional amounts that are identified during the measurement period are recorded in the reporting period in which the adjustment amounts are determined. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings.
Apartment Finder [Member]
Business Acquisition [Line Items]
Acquisitions	The Company applied the acquisition method to account for the Apartment Finder transaction, which requires that, among other things, assets acquired and liabilities assumed be recorded at their fair values as of the acquisition date.

INVESTMENTS (Policies)

INVESTMENTS (Policies)	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Investments	Investments The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as auction rate securities. Investments are carried at fair value. Changes in unrealized holding gains and losses, net of the related tax effect, on available-for-sale securities are excluded from earnings and are reported as a separate component of accumulated other comprehensive loss in stockholders’ equity until realized. A decline in market value of any available-for-sale security below cost that is deemed to be other-than-temporary results in a reduction in carrying amount to fair value. The impairment is charged to earnings and a new cost basis for the security is established. Dividend and interest income are recognized when earned. Realized gains and losses from the sale of available-for-sale securities are determined on a specific-identification basis. The Company determines the appropriate classification of debt and equity investments at the time of purchase and re-evaluates such designation as of each balance sheet date. The Company considers all of its investments to be available-for-sale. The Company's investments consist of long-term variable rate debt instruments with an auction reset feature, referred to as ARS. Investments are carried at fair value.

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Policies)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and contingencies	Currently, and from time to time, the Company is involved in litigation incidental to the conduct of its business. In accordance with GAAP, the Company records a provision for a liability when it is both probable that a liability has been incurred and the amount can be reasonably estimated.

NET INCOME (LOSS) PER SHARE (Po

NET INCOME (LOSS) PER SHARE (Policies)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	Net Income (Loss) Per Share Net income (loss) per share is computed by dividing net income (loss) by the weighted average number of common shares outstanding during the period on a basic and diluted basis. The Company’s potentially dilutive securities include stock options and restricted stock. Diluted net income (loss) per share considers the impact of potentially dilutive securities except in periods in which there is a net loss, as the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Stock options to purchase approximately 87,000 and 194,000 shares that were outstanding for the years ended December 31, 2017 and December 31, 2016 , respectively, were not included in the computation of diluted net income per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. The Company did not consider the impact of potentially dilutive securities for the year ended December 31, 2015 when calculating the diluted net loss per share because the inclusion of the potentially dilutive common shares would have an anti-dilutive effect. Shares underlying restricted common stock awards that vest based on Company performance and service conditions that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share. Shares underlying restricted stock units that vest based on Company service conditions, that have not been achieved as of the end of the period are not included in the computation of basic or diluted earnings per share.

EMPLOYEE BENEFIT PLANS (Policie

EMPLOYEE BENEFIT PLANS (Policies)	12 Months Ended
	Dec. 31, 2017
Compensation Related Costs [Abstract]
Stock-Based Compensation	Stock-Based Compensation Equity instruments issued in exchange for services performed by officers, employees, and directors of the Company are accounted for using a fair-value based method and the fair value of such equity instruments is recognized as expense in the consolidated statements of operations. Stock-based compensation expense is measured at the grant date of the stock-based awards that vest over set time periods based on their fair values, and is recognized on a straight-line basis as expense over the vesting periods of the awards, net of an estimated forfeiture rate. For equity instruments that vest based on performance, the Company assesses the probability of the achievement of the performance conditions at the end of each reporting period, or more frequently based upon the occurrence of events that may change the probability of whether the performance conditions would be met. If the Company's initial estimates of the achievement of the performance conditions change, the related stock-based compensation expense and timing of recognition may fluctuate from period to period based on those estimates. For equity instruments that vest based on a performance condition and a market condition, the Company estimates the fair value of each equity instrument granted on the date of grant using a Monte-Carlo simulation model. This pricing model uses multiple simulations to evaluate the probability of achieving the market condition to calculate the fair value of the awards. Stock-based compensation expense is updated based on the expected achievement of the related performance conditions at the end of each reporting period. If the performance conditions are not met, no stock-based compensation expense will be recognized, and any previously recognized stock-based compensation expense will be reversed. The Company estimates the fair value of its performance-based restricted common stock awards with a market condition on the date of grant using a Monte-Carlo simulation valuation model. This pricing model uses multiple simulations to evaluate the probability of the Company achieving various stock price levels to determine the expected TSR performance ranking. Expense is only recorded for awards that are expected to vest, net of estimated forfeitures.

SUMMARY OF SIGNIFICANT ACCOUN32

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule of accumulated other comprehensive loss	The components of accumulated other comprehensive loss were as follows (in thousands): As of December 31, 2017 2016 Foreign currency translation adjustment $ (8,290 ) $ (12,191 ) Net unrealized loss on investments, net of tax (730 ) (848 ) Total accumulated other comprehensive loss $ (9,020 ) $ (13,039 )
Stock-based compensation expense for stock options and restricted stock	Stock-based compensation expense for stock options and restricted stock issued under equity incentive plans and stock purchases under the ESPP included in the Company’s results of operations were as follows (in thousands): Year Ended December 31, 2017 2016 2015 Cost of revenues $ 4,971 $ 5,495 $ 5,815 Selling and marketing (excluding customer base amortization) 7,086 6,634 5,114 Software development 7,071 6,546 5,712 General and administrative 19,902 17,674 17,896 Total stock-based compensation $ 39,030 $ 36,349 $ 34,537
Schedule of property and equipment	Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets: Leasehold improvements Shorter of lease term or useful life Furniture and office equipment Five to ten years Vehicles Five to ten years Computer hardware and software Two to five years Property and equipment consists of the following (in thousands): December 31, 2017 2016 Leasehold improvements $ 59,447 $ 53,073 Furniture, office equipment and vehicles 52,163 45,035 Computer hardware and software 71,281 64,577 Property and equipment, gross 182,891 162,685 Accumulated depreciation and amortization (98,395 ) (75,117 ) Property and equipment, net $ 84,496 $ 87,568

ACQUISITION (Tables)

ACQUISITION (Tables)	12 Months Ended
	Dec. 31, 2017
Apartment Finder [Member]
Business Acquisition [Line Items]
Schedule of assets acquired and liabilities assumed	The following table summarizes the amounts for acquired assets and liabilities recorded at their fair values as of the acquisition date (in thousands): Cash and cash equivalents $ 39 Accounts receivable 4,556 Goodwill 107,692 Acquired trade names and other intangible assets 23,642 Acquired customer base 21,856 Acquired database technology 4,076 Acquired building photography 2,425 Deferred income taxes, net 9,290 Other assets and liabilities (849 ) Fair value of identifiable net assets acquired $ 172,727

INVESTMENTS (Tables)

INVESTMENTS (Tables)	12 Months Ended
	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Scheduled maturities of investments classified as available-for-sale	Scheduled maturities of investments classified as available-for-sale as of December 31, 2017 are as follows (in thousands): Maturity Fair Value Due in: 2018 $ — 2019 — 2022 — 2023 — 2027 — 2028 and thereafter 10,070 Available-for-sale investments $ 10,070
Schedule of available for sale securities reconciliation	As of December 31, 2017 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 10,800 $ — $ (730 ) $ 10,070 Available-for-sale investments $ 10,800 $ — $ (730 ) $ 10,070 As of December 31, 2016 , the amortized cost basis and fair value of investments classified as available-for-sale were as follows (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Auction rate securities $ 10,800 $ — $ (848 ) $ 9,952 Available-for-sale investments $ 10,800 $ — $ (848 ) $ 9,952
Schedule of unrealized loss on investments	The components of the Company’s investments in an unrealized loss position for twelve months or longer were as follows (in thousands): December 31, 2017 2016 Aggregate Fair Value Gross Unrealized Losses Aggregate Fair Value Gross Unrealized Losses Auction rate securities $ 10,070 $ (730 ) $ 9,952 $ (848 ) Investments in an unrealized loss position $ 10,070 $ (730 ) $ 9,952 $ (848 )

FAIR VALUE (Tables)

FAIR VALUE (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Summary of fair value hierarchy for the company's financial assets and liabilities measured at fair value on a recurring basis	The following table represents the Company's fair value hierarchy for its financial assets (cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2017 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money markets $ 586,084 $ — $ — $ 586,084 Commercial paper 351,098 — — 351,098 Auction rate securities — — 10,070 10,070 Total assets measured at fair value $ 937,182 $ — $ 10,070 $ 947,252 The carrying value of accounts receivable, accounts payable, accrued expenses and long-term debt approximates fair value. The following table represents the Company's fair value hierarchy for its financial assets (cash equivalents and investments) measured at fair value on a recurring basis as of December 31, 2016 (in thousands): Level 1 Level 2 Level 3 Total Assets: Money markets $ 175,344 $ — $ — $ 175,344 Commercial paper 6,383 — — 6,383 Auction rate securities — — 9,952 9,952 Total assets measured at fair value $ 181,727 $ — $ 9,952 $ 191,679
Summary of changes in the fair value of the company's level 3 assets	The following table summarizes changes in fair value of the Company’s Level 3 assets from December 31, 2015 to December 31, 2017 (in thousands): Auction Rate Securities Balance at December 31, 2015 $ 15,507 Decrease in unrealized loss included in accumulated other comprehensive loss 395 Settlements (5,950 ) Balance at December 31, 2016 9,952 Decrease in unrealized loss included in accumulated other comprehensive loss 118 Balance at December 31, 2017 $ 10,070

PROPERTY AND EQUIPMENT (Tables)

PROPERTY AND EQUIPMENT (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Schedule of property and equipment	Property and equipment are stated at cost, net of accumulated depreciation and amortization. All repairs and maintenance costs are expensed as incurred. Depreciation and amortization are calculated on a straight-line basis over the following estimated useful lives of the assets: Leasehold improvements Shorter of lease term or useful life Furniture and office equipment Five to ten years Vehicles Five to ten years Computer hardware and software Two to five years Property and equipment consists of the following (in thousands): December 31, 2017 2016 Leasehold improvements $ 59,447 $ 53,073 Furniture, office equipment and vehicles 52,163 45,035 Computer hardware and software 71,281 64,577 Property and equipment, gross 182,891 162,685 Accumulated depreciation and amortization (98,395 ) (75,117 ) Property and equipment, net $ 84,496 $ 87,568

GOODWILL (Tables)

GOODWILL (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill [Abstract]
Schedule of goodwill	The changes in the carrying amount of goodwill by operating segment consist of the following (in thousands): North America International Total Goodwill, December 31, 2015 $ 1,227,310 $ 25,635 $ 1,252,945 Acquisition 467 5,933 6,400 Effect of foreign currency translation — (4,479 ) (4,479 ) Goodwill, December 31, 2016 1,227,777 27,089 1,254,866 Acquisitions 25,717 — 25,717 Effect of foreign currency translation — 2,874 2,874 Goodwill, December 31, 2017 $ 1,253,494 $ 29,963 $ 1,283,457

INTANGIBLE ASSETS (Tables)

INTANGIBLE ASSETS (Tables)	12 Months Ended
	Dec. 31, 2017
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Schedule of acquired finite-lived intangible assets by major class	Intangible assets consist of the following (in thousands, except amortization period data): December 31, Weighted- Average Amortization Period (in years) 2017 2016 Capitalized product development cost $ 2,275 $ 2,275 4 Accumulated amortization (2,262 ) (2,217 ) Capitalized product development cost, net 13 58 Building photography 18,739 17,271 4 Accumulated amortization (18,212 ) (16,351 ) Building photography, net 527 920 Acquired database technology 83,469 78,151 5 Accumulated amortization (79,188 ) (72,691 ) Acquired database technology, net 4,281 5,460 Acquired customer base 225,879 220,749 10 Accumulated amortization (169,157 ) (150,445 ) Acquired customer base, net 56,722 70,304 Acquired trade names and other intangible assets 167,718 153,607 13 Accumulated amortization (46,369 ) (34,384 ) Acquired trade names and other intangible assets, net 121,349 119,223 Intangible assets, net $ 182,892 $ 195,965

LONG-TERM DEBT (Tables)

LONG-TERM DEBT (Tables)	12 Months Ended
	Dec. 31, 2017
Long-term Debt, Current and Noncurrent [Abstract]
Components of long-term debt	The following table represents the Company's long-term debt at December 31, 2016 (in thousands): December 31, 2016 Term loan $ 345,000 Debt issuance costs, net (6,661 ) Total debt 338,339 Current maturities of long-term debt (35,000 ) Current debt issuance costs, net 3,134 Total long-term debt, less current portion $ 306,473

INCOME TAXES (Tables)

INCOME TAXES (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of components of income tax expense	The components of the provision for income taxes attributable to operations consist of the following (in thousands): Year Ended December 31, 2017 2016 2015 Current: Federal $ 41,453 $ 32,198 $ 10,295 State 3,518 3,682 1,503 Foreign 295 76 40 Total current 45,266 35,956 11,838 Deferred: Federal (7,917 ) 12,586 (8,382 ) State 4,695 3,014 2,590 Foreign 319 35 — Total deferred (2,903 ) 15,635 (5,792 ) Total provision for income taxes $ 42,363 $ 51,591 $ 6,046
Schedule of deferred tax assets and liabilities	The components of deferred tax assets and liabilities consist of the following (in thousands): December 31, 2017 2016 Deferred tax assets: Reserve for bad debts $ 1,636 $ 2,437 Accrued compensation 6,706 5,562 Stock compensation 10,568 14,268 Net operating losses 25,899 30,319 Accrued reserve and other 1,393 2,097 Unrealized loss on securities 185 326 Deferred rent 6,533 7,814 Deferred gain on the sale of building 4,741 8,166 Total deferred tax assets, prior to valuation allowance 57,661 70,989 Valuation allowance (13,032 ) (8,557 ) Total deferred tax assets, net of valuation allowance 44,629 62,432 Deferred tax liabilities: Prepaids (1,239 ) (1,753 ) Depreciation (6,229 ) (13,045 ) Intangibles (43,800 ) (58,747 ) Total deferred tax liabilities (51,268 ) (73,545 ) Net deferred tax assets (liabilities) $ (6,639 ) $ (11,113 )
Schedule of effective income tax rate reconciliation	9. INCOME TAXES — (CONTINUED) The Company’s provision for income taxes resulted in effective tax rates that varied from the statutory federal income tax rate as follows (in thousands): Year Ended December 31, 2017 2016 2015 Expected federal income tax provision at statutory rate $ 57,770 $ 47,832 $ 903 State income taxes, net of federal benefit 4,776 3,638 (678 ) Foreign income taxes, net effect (3,540 ) (31 ) 469 Increase (decrease) in valuation allowance 3,624 (103 ) 1,956 Nondeductible compensation 230 141 574 Nondeductible transaction costs — 103 229 Meals and entertainment 958 712 1,032 Tax rate changes (7,340 ) 283 1,203 Research credits (20,547 ) (920 ) — Excess tax benefit (7,010 ) — — Tax reserves 12,646 (150 ) 71 Other adjustments 796 86 287 Income tax expense $ 42,363 $ 51,591 $ 6,046
Schedule of unrecognized tax benefits	The following table summarizes the activity related to the Company’s unrecognized tax benefits (in thousands): Unrecognized tax benefits as of December 31, 2014 $ 5,749 Increase for current year tax positions — Increase for prior year tax positions 1,954 Expiration of the statute of limitation for assessment of taxes (39 ) Unrecognized tax benefits as of December 31, 2015 7,664 Increase for current year tax positions 368 Decrease for prior year tax positions (6,115 ) Expiration of the statute of limitation for assessment of taxes (74 ) Unrecognized tax benefits as of December 31, 2016 1,843 Increase for current year tax positions 12,620 Decrease for prior year tax positions (34 ) Expiration of the statute of limitation for assessment of taxes (66 ) Unrecognized tax benefits as of December 31, 2017 $ 14,363

COMMITMENTS AND CONTINGENCIES41

COMMITMENTS AND CONTINGENCIES (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Future minimum lease payments	Future minimum lease payments as of December 31, 2017 are as follows (in thousands): 2018 $ 30,853 2019 27,925 2020 26,136 2021 24,544 2022 22,883 Thereafter 46,061 Total future minimum lease payments $ 178,402

SEGMENT REPORTING (Tables)

SEGMENT REPORTING (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Summarized information by operating segment	Summarized information by operating segment consists of the following (in thousands): Year Ended December 31, 2017 2016 2015 Revenues North America $ 934,464 $ 809,492 $ 686,573 International External customers 30,766 28,138 25,191 Intersegment revenue 42 40 41 Total International revenue 30,808 28,178 25,232 Intersegment eliminations (42 ) (40 ) (41 ) Total revenues $ 965,230 $ 837,630 $ 711,764 EBITDA North America $ 236,906 $ 210,901 $ 87,092 International 553 4,169 2,895 Total EBITDA $ 237,459 $ 215,070 $ 89,987
Reconciliation of net income (loss) to EBITDA	The reconciliation of net income (loss) to EBITDA consists of the following (in thousands): Year Ended December 31, 2017 2016 2015 Net income (loss) $ 122,695 $ 85,071 $ (3,465 ) Amortization of acquired intangible assets in cost of revenues 19,707 22,819 30,077 Amortization of acquired intangible assets in operating expenses 17,684 22,731 27,931 Depreciation and other amortization 26,252 24,615 20,524 Interest and other income (4,044 ) (1,773 ) (537 ) Interest and other expense 9,014 10,016 9,411 Loss on debt extinguishment 3,788 — — Income tax expense 42,363 51,591 6,046 EBITDA $ 237,459 $ 215,070 $ 89,987
Summarized information by operating segment, assets and liabilities	Summarized information by operating segment consists of the following (in thousands): December 31, 2017 2016 Property and equipment, net North America $ 79,736 $ 84,727 International 4,760 2,841 Total property and equipment, net $ 84,496 $ 87,568 Goodwill North America $ 1,253,494 $ 1,227,777 International 29,963 27,089 Total goodwill $ 1,283,457 $ 1,254,866 Assets North America $ 2,816,156 $ 2,139,896 International 57,285 45,167 Total assets $ 2,873,441 $ 2,185,063 Liabilities North America $ 201,831 $ 520,833 International 20,360 10,017 Total liabilities $ 222,191 $ 530,850
Revenues by services	The revenues by type of service consists of the following (in thousands): Year Ended December 31, 2017 2016 2015 Information and analytics CoStar Suite (1) $ 463,185 $ 408,456 $ 360,440 Information services (2) 72,618 77,178 75,790 Online marketplaces Multifamily (3) 279,855 224,835 160,630 Commercial property and land (4) 149,572 127,161 114,904 Total revenues $ 965,230 $ 837,630 $ 711,764 (1) CoStar Suite is comprised of: CoStar Property Professional, CoStar COMPS Professional, CoStar Tenant; and CoStar Portfolio Strategy. (2) Information services is comprised of: LoopNet Premium Searcher; CoStar Real Estate Manager; CoStar Risk Analytics COMPASS; CoStar Investment Analysis Portfolio Maximizer; CoStar Investment Analysis Request; CoStar Brokerage Applications; PROPEX; Grecam; Belbex; and Thomas Daily. (3) Multifamily is comprised of Apartments.com, ApartmentFinder.com and ApartmentHomeLiving.com; WestsideRentals.com; Apartamentos.com; and The Screening Pros. (4) Commercial property and land is comprised of: LoopNet Premium Lister; LoopLink; CoStar Advertising; BizBuySell and BizQuest; LandsofAmerica,LandAndFarm, and LandWatch; and CoStar Private Sale Network.

NET INCOME (LOSS) PER SHARE (Ta

NET INCOME (LOSS) PER SHARE (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Calculation of basic and diluted net income (loss) per share	The following table sets forth the calculation of basic and diluted net income (loss) per share (in thousands except per share data): Year Ended December 31, 2017 2016 2015 Numerator: Net income (loss) $ 122,695 $ 85,071 $ (3,465 ) Denominator: Denominator for basic net income (loss) per share — weighted-average outstanding shares 33,200 32,167 31,950 Effect of dilutive securities: Stock options and restricted stock 359 269 — Denominator for diluted net income (loss) per share — weighted-average outstanding shares 33,559 32,436 31,950 Net income (loss) per share — basic $ 3.70 $ 2.64 $ (0.11 ) Net income (loss) per share — diluted $ 3.66 $ 2.62 $ (0.11 )
Schedule of anti-dilutive securities excluded from computation of earnings per share	The following table summarizes the shares underlying the performance-based restricted stock awards and service-based restricted stock units excluded from the basic and diluted calculation (in thousands): Year Ended December 31, 2017 2016 2015 Performance-based restricted stock awards 58 59 55 Service-based restricted stock units 1 1 1 Total shares excluded from computation 59 60 56

EMPLOYEE BENEFIT PLANS (Tables)

EMPLOYEE BENEFIT PLANS (Tables)	12 Months Ended
	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Schedule of option activity	Option activity was as follows: Number of Shares Range of Exercise Price Weighted- Average Exercise Price Weighted- Average Remaining Contract Life (in years) Aggregate Intrinsic Value (in thousands) Outstanding at December 31, 2014 370,179 $36.48 - $201.04 $ 99.12 Granted 89,500 $193.69 - $193.69 $ 193.69 Exercised (59,602 ) $36.48 - $201.04 $ 85.48 Outstanding at December 31, 2015 400,077 $36.48 - $201.04 $ 122.30 Granted 82,400 $182.75 - $182.75 $ 182.75 Exercised (29,285 ) $36.48 - $201.04 $ 112.78 Canceled or expired (13,034 ) $193.69 - $201.04 $ 195.78 Outstanding at December 31, 2016 440,158 $36.48 - $201.04 $ 132.08 Granted 95,500 $204.91 $ 204.91 Exercised (81,815 ) $36.48 - $201.04 $ 83.07 Outstanding at December 31, 2017 453,843 $36.73 - $204.91 $ 156.24 6.67 $ 63,861 Exercisable at December 31, 2015 220,107 $36.48 - $201.04 $ 77.63 Exercisable at December 31, 2016 284,489 $36.48 - $201.04 $ 100.94 Exercisable at December 31, 2017 278,239 $36.73 - $201.04 $ 130.91 5.47 $ 46,199
Summarized information regarding options outstanding	The following table summarizes information regarding options outstanding at December 31, 2017 : Options Outstanding Options Exercisable Range of Exercise Price Number of Shares Weighted-Average Remaining Contractual Life (in years) Weighted- Average Exercise Price Number of Shares Weighted- Average Exercise Price $36.73 - $58.06 41,359 3.13 $ 56.64 41,359 $ 56.64 $58.07 - $59.59 40,004 4.14 $ 58.95 40,004 $ 58.95 $59.60 - $81.19 1,000 3.42 $ 60.23 1,000 $ 60.23 $81.20 - $142.45 70,944 5.19 $ 102.16 70,944 $ 102.16 $142.46 - $188.22 76,934 8.19 $ 182.75 21,999 $ 182.75 $188.23 - $197.37 65,802 7.17 $ 193.69 40,633 $ 193.69 $197.38 - $202.98 62,300 6.16 $ 201.04 62,300 $ 201.04 $202.99 - $204.91 95,500 9.16 $ 204.91 — $ — $36.73 - $204.91 453,843 6.67 $ 156.24 278,239 $ 130.91
Unvested restricted stock awards activity	The following table presents unvested restricted stock awards activity for the year ended December 31, 2017 : Restricted Stock Awards — without Market Condition Restricted Stock Awards — with Market Condition Number of Shares Weighted-Average Grant Date Fair Value per Share Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock awards at December 31, 2016 394,712 $ 192.38 72,480 $ 202.07 Granted 156,238 $ 211.69 32,160 $ 218.59 Vested (156,240 ) $ 177.72 (13,477 ) $ 216.20 Canceled (30,827 ) $ 196.87 (5,963 ) $ 216.20 Unvested restricted stock awards at December 31, 2017 363,883 $ 206.59 85,200 $ 205.08
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value assumption for options granted	The Company estimated the fair value of each option granted on the date of grant using the Black-Scholes option-pricing model, using the assumptions in the following table: Year Ended December 31, 2017 2016 2015 Dividend yield 0 % 0 % 0 % Expected volatility 28 % 31 % 30 % Risk-free interest rate 2 % 1 % 2 % Expected life (in years) 5 5 5
Restricted Stock Awards — with Market Condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value assumption for options granted	The assumptions used to estimate the fair value of performance-based restricted common stock awards with a market condition granted were as follows: Year Ended December 31, 2017 2016 2015 Dividend yield 0 % 0 % 0 % Expected volatility 28 % 28 % 26 % Risk-free interest rate 2 % 1 % 1 % Expected life (in years) 3 3 3 Weighted-average grant date fair value $ 218.59 $ 184.97 $ 208.08
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted stock awards activity	The following table presents unvested restricted stock units activity for the year ended December 31, 2017 : Number of Shares Weighted-Average Grant Date Fair Value per Share Unvested restricted stock units at December 31, 2016 1,199 $ 182.64 Granted 69 $ 291.82 Vested (306 ) $ 183.28 Canceled — $ — Unvested restricted stock units at December 31, 2017 962 $ 190.27

QUARTERLY RESULTS (Tables)

QUARTERLY RESULTS (Tables)	12 Months Ended
	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Quarterly Financial Information [Table Text Block]	The following is a summary of the unaudited quarterly results of operations for the years ended December 31, 2017 and 2016 . 2017 Mar. 31 Jun. 30 Sep. 30 Dec. 31 Revenues $ 226,553 $ 237,153 $ 247,533 $ 253,991 Cost of revenues 51,346 55,273 55,483 58,301 Gross profit 175,207 181,880 192,050 195,690 Operating expenses 137,545 153,997 134,537 144,932 Income from operations 37,662 27,883 57,513 50,758 Interest and other income 429 605 555 2,455 Interest and other expense (2,686 ) (2,693 ) (2,901 ) (734 ) Loss on debt extinguishment — — — (3,788 ) Income before income taxes 35,405 25,795 55,167 48,691 Income tax expense 13,275 3,611 20,990 4,487 Net income $ 22,130 $ 22,184 $ 34,177 $ 44,204 Net income per share — basic $ 0.69 $ 0.68 $ 1.05 $ 1.24 Net income per share — diluted $ 0.68 $ 0.68 $ 1.04 $ 1.22 2016 Mar. 31 Jun. 30 Sep. 30 Dec. 31 Revenues $ 199,739 $ 206,869 $ 212,711 $ 218,311 Cost of revenues 42,900 42,679 42,222 46,013 Gross profit 156,839 164,190 170,489 172,298 Operating expenses 126,538 136,071 130,893 125,409 Income from operations 30,301 28,119 39,596 46,889 Interest and other income 84 159 344 1,186 Interest and other expense (2,509 ) (2,455 ) (2,498 ) (2,554 ) Income before income taxes 27,876 25,823 37,442 45,521 Income tax expense 11,155 10,247 14,241 15,948 Net income $ 16,721 $ 15,576 $ 23,201 $ 29,573 Net income per share — basic $ 0.52 $ 0.48 $ 0.72 $ 0.92 Net income per share — diluted $ 0.52 $ 0.48 $ 0.72 $ 0.91

ORGANIZATION (Details)

ORGANIZATION (Details)	12 Months Ended
	Dec. 31, 2017operating_segments
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of business segments (in segments)	2
Term of subscription-based license agreements (in years)	1 year

SUMMARY OF SIGNIFICANT ACCOUN47

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - revenue Recognition and Advertising Costs) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Term of subscription-based license agreements (in years)	1 year
Advertising costs	$ 104	$ 109	$ 132

SUMMARY OF SIGNIFICANT ACCOUN48

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Foreign Currency Translation and Accumulated Other Comprehensive Loss) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) Net of Tax [Abstract]
Material gains or losses from foreign currency exchange transactions	$ 0	$ 0	$ 0
Foreign currency translation adjustment	(8,290,000)	(12,191,000)
Net unrealized loss on investments, net of tax	(730,000)	(848,000)
Total accumulated other comprehensive loss	(9,020,000)	(13,039,000)
Reclassifications out of accumulated other comprehensive loss	$ 0	$ (808,000)	$ 0

SUMMARY OF SIGNIFICANT ACCOUN49

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Stock-Based Compensation) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-Based Compensation Expense [Abstract]
Compensation expense	$ 39,030	$ 36,349	$ 34,537
Performance service period	3 years
Net cash proceeds from the exercise of stock options and ESPP	$ 9,888	5,861	7,404
Excess tax benefit from stock-based compensation		4,698	8,528
Cost of revenues
Stock-Based Compensation Expense [Abstract]
Compensation expense	4,971	5,495	5,815
Selling and marketing
Stock-Based Compensation Expense [Abstract]
Compensation expense	7,086	6,634	5,114
Software development
Stock-Based Compensation Expense [Abstract]
Compensation expense	7,071	6,546	5,712
General and administrative
Stock-Based Compensation Expense [Abstract]
Compensation expense	19,902	17,674	17,896
Restricted Stock Awards — with Market Condition
Stock-Based Compensation Expense [Abstract]
Compensation expense	5,000	$ 3,000	$ 3,000
Compensation cost expected to be recognized in future years	$ 6,000

SUMMARY OF SIGNIFICANT ACCOUN50

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Cash and Cash Equivalents and Concentration of Credit Risk)	12 Months Ended
	Dec. 31, 2017
Cash and Cash Equivalents [Abstract]
Number of customers accounting for more than 5% of revenue (in customers)	No single customer accounted for more than 5% of the Company’s revenues for each of the years ended December 31, 2017, 2016, and 2015.
Maximum original maturity of highly liquid investments to be considered cash equivalents	The Company considers all highly liquid investments purchased with an original maturity of three months or less to be cash equivalents

SUMMARY OF SIGNIFICANT ACCOUN51

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Property and Equipment)	12 Months Ended
	Dec. 31, 2017
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	Shorter of lease term or useful life
Furniture and Office Equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Furniture and Office Equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Vehicles [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years
Vehicles [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	10 years
Computer Hardware and Software [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	2 years
Computer Hardware and Software [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, estimated useful life (in years)	5 years

SUMMARY OF SIGNIFICANT ACCOUN52

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Goodwill and Intangible Assets)	12 Months Ended
	Dec. 31, 2017
Acquired Database Technology [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	3 years
Acquired Database Technology [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	8 years
Acquired Trade Names and Other Intangible Assets [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	3 years
Acquired Trade Names and Other Intangible Assets [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	15 years
Acquired Customer Base [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	10 years
Acquired Customer Base [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	13 years
Acquired Building Photography [Member] | Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	3 years
Acquired Building Photography [Member] | Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible asset, useful life (in years)	5 years

SUMMARY OF SIGNIFICANT ACCOUN53

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details - Debt Issuance Costs and Recent Accounting Pronouncements) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Capitalized debt issuance costs, net	$ 4,208	$ 6,661
Amortization of debt issuance costs	$ 2,000	$ 3,000	$ 3,000

SUMMARY OF SIGNIFICANT ACCOUN54

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard				$ 2,162
Deferred income taxes, net		$ 7,273		5,431
Excess Tax Benefit from Share-based Compensation, Operating Activities		$ 5,000	$ 9,000
Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred income taxes, net				(2,000)
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Sales Commission, Amortization Period	3 years
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard				2,162
Retained Earnings [Member] | Accounting Standards Update 2016-09, Excess Tax Benefit Component [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of new accounting standard				$ 2,000
Subsequent Event [Member] | Minimum [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Sales Commission	$ 48,000
Subsequent Event [Member] | Maximum [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Sales Commission	$ 58,000

ACQUISITION (Details for Apartm

ACQUISITION (Details for Apartment Finder)	Jun. 01, 2015USD ($)distinct_intangible_asset	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Business Acquisition [Line Items]
Goodwill		$ 1,283,457,000	$ 1,254,866,000	$ 1,252,945,000
Goodwill tax deductible amount		$ 24,000,000	$ 0
Apartment Finder [Member]
Business Acquisition [Line Items]
Percentage of outstanding stock acquired	100.00%
Payments to acquire businesses	$ 173,000,000
Closing purchase price adjustments	3,000,000
Post closing purchase price adjustments	21,000
Goodwill	107,692,000
Goodwill tax deductible amount	$ 0
Acquired Trade Names and Other Intangible Assets [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		13 years
Acquired Trade Names and Other Intangible Assets [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	9 years
Acquired Customer Base [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		10 years
Acquired Customer Base [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Number of distinct intangible assets | distinct_intangible_asset	3
Estimated useful life of acquired assets	10 years
Acquired Database Technology [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		5 years
Acquired Database Technology [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	5 months
Acquired Building Photography [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets		4 years
Acquired Building Photography [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Estimated useful life of acquired assets	5 months

ACQUISITION ACQUISITION (Detail

ACQUISITION ACQUISITION (Details - Schedule of Assets Acquired and Liabilities Assumed for Apartment Finder) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 01, 2015
Business Acquisition [Line Items]
Goodwill	$ 1,283,457	$ 1,254,866	$ 1,252,945
Apartment Finder [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				$ 39
Accounts receivable				4,556
Goodwill				107,692
Deferred income taxes, net				9,290
Other assets and liabilities				(849)
Fair value of identifiable net assets acquired				172,727
Acquired Trade Names and Other Intangible Assets [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill				23,642
Acquired Customer Base [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill				21,856
Acquired Database Technology [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill				4,076
Acquired Building Photography [Member] | Apartment Finder [Member]
Business Acquisition [Line Items]
Intangible assets, other than goodwill				$ 2,425

INVESTMENTS, SCHEDULED MATURITI

INVESTMENTS, SCHEDULED MATURITIES AND REALIZED GAINS AND LOSSES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities, gross realized gains	$ 0	$ 808,000	$ 0
Proceeds from sales of available-for-sale securities	1,000,000
Available-for-sale securities, gross realized losses	0	0	$ 0
Debt Maturities Fair Value [Abstract]
2,017	0
2019 — 2022	0
2023 — 2027	0
2028 and thereafter	10,070,000
Available-for-sale investments	$ 10,070,000	$ 9,952,000

INVESTMENTS, AVAILABLE-FOR-SALE

INVESTMENTS, AVAILABLE-FOR-SALE SECURITIES (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	$ 10,800,000	$ 10,800,000
Gross unrealized gains	0	0
Gross unrealized losses	(730,000)	(848,000)
Fair Value	10,070,000	9,952,000
Available-for-sale securities, unrealized loss positions
Continuous unrealized loss position, 12 months or more, aggregated fair value	10,070,000	9,952,000
Continuous unrealized loss position, 12 months or more, gross unrealized losses	(730,000)	(848,000)
Continuous unrealized loss position, less than 12 months	0	0
Auction Rate Securities [Member]
Available-for-sale Securities Reconciliation [Abstract]
Amortized cost	10,800,000	10,800,000
Gross unrealized gains	0	0
Gross unrealized losses	(730,000)	(848,000)
Fair Value	10,070,000	9,952,000
Available-for-sale securities, unrealized loss positions
Continuous unrealized loss position, 12 months or more, aggregated fair value	10,070,000	9,952,000
Continuous unrealized loss position, 12 months or more, gross unrealized losses	$ (730,000)	$ (848,000)

FAIR VALUE (Details)

FAIR VALUE (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Unobservable inputs assets (level 3) [Roll forward]
Temporary impairment of the auction rates security investments	$ (730)	$ (848)
Fair Value, Measurements, Recurring [Member]
Assets:
Total assets measured at fair value	947,252	191,679
Fair Value, Measurements, Recurring [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	586,084	175,344
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	351,098	6,383
Fair Value, Measurements, Recurring [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	10,070	9,952
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Assets:
Total assets measured at fair value	937,182	181,727
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	586,084	175,344
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	351,098	6,383
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member]
Assets:
Total assets measured at fair value	10,070	9,952
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Money Market Funds [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Commercial Paper [Member]
Assets:
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Auction Rate Securities [Member]
Assets:
Total assets measured at fair value	10,070	9,952
Auction Rate Securities [Member]
Unobservable inputs assets (level 3) [Roll forward]
Beginning balance	9,952	15,507
Decrease in unrealized loss included in accumulated other comprehensive loss	118	395
Settlements		(5,950)
Ending balance	$ 10,070	$ 9,952
Auction rate securities variable rate debt instruments interest rate reset period	28 days
The minimum contractual maturities on the underlying securities involved in the auction rate securities (in years)	20 years
Par value of company held auction rate securities	$ 11,000
Discount rate (in percent)	6.00%	5.00%
Temporary impairment of the auction rates security investments	$ (730)

PROPERTY AND EQUIPMENT (Details

PROPERTY AND EQUIPMENT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 182,891	$ 162,685
Accumulated depreciation and amortization	(98,395)	(75,117)
Property and equipment, net	84,496	87,568
Depreciation expense for property and equipment	26,000	24,000	$ 20,000
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	59,447	53,073
Furniture, Office Equipment and Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	52,163	45,035
Computer Hardware and Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 71,281	$ 64,577

GOODWILL (Details)

GOODWILL (Details) - USD ($)	Jul. 18, 2017	May 10, 2017	Jan. 31, 2017	May 03, 2016	Dec. 31, 2017	Dec. 31, 2016	Jun. 01, 2015
Goodwill [Roll Forward]
Goodwill, beginning balance					$ 1,254,866,000	$ 1,252,945,000
Acquisitions					25,717,000	6,400,000
Effect of foreign currency translation					2,874,000	(4,479,000)
Goodwill, ending balance					1,283,457,000	1,254,866,000
Goodwill tax deductible amount					24,000,000	0
The Screening Pros, LLC [Member]
Goodwill [Roll Forward]
Acquisitions	$ 2,000,000
Apartment Finder [Member]
Goodwill [Roll Forward]
Goodwill tax deductible amount							$ 0
Independent Distributor Buyout [Member]
Goodwill [Roll Forward]
Acquisitions						467,000
Thomas Daily [Member]
Goodwill [Roll Forward]
Acquisitions				$ 6,000,000
Westside Rentals [Member]
Goodwill [Roll Forward]
Acquisitions			$ 8,000,000
LandWatch [Member]
Goodwill [Roll Forward]
Acquisitions		$ 15,000,000
North America [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance					1,227,777,000	1,227,310,000
Acquisitions					25,717,000	467,000
Effect of foreign currency translation					0	0
Goodwill, ending balance					1,253,494,000	1,227,777,000
International [Member]
Goodwill [Roll Forward]
Goodwill, beginning balance					27,089,000	25,635,000
Acquisitions					0	5,933,000
Effect of foreign currency translation					2,874,000	(4,479,000)
Goodwill, ending balance					$ 29,963,000	$ 27,089,000

INTANGIBLE ASSETS (Details)

INTANGIBLE ASSETS (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, net	$ 182,892	$ 195,965
Amortization of intangible assets	37,000	46,000	$ 59,000
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Amortization expense for 2017	29,000
Amortization expense for 2018	24,000
Amortization expense for 2019	21,000
Amortization expense for 2020	20,000
Amortization expense for 2021	17,000
Capitalized Product Development Costs [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	2,275	2,275
Finite-lived intangible assets, accumulated amortization	(2,262)	(2,217)
Finite-lived intangible assets, net	$ 13	58
Weighted-average amortization period (in years}	4 years
Acquired Building Photography [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 18,739	17,271
Finite-lived intangible assets, accumulated amortization	(18,212)	(16,351)
Finite-lived intangible assets, net	$ 527	920
Weighted-average amortization period (in years}	4 years
Acquired Database Technology [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 83,469	78,151
Finite-lived intangible assets, accumulated amortization	(79,188)	(72,691)
Finite-lived intangible assets, net	$ 4,281	5,460
Weighted-average amortization period (in years}	5 years
Acquired Customer Base [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 225,879	220,749
Finite-lived intangible assets, accumulated amortization	(169,157)	(150,445)
Finite-lived intangible assets, net	$ 56,722	70,304
Weighted-average amortization period (in years}	10 years
Acquired Trade Names and Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived intangible assets, gross	$ 167,718	153,607
Finite-lived intangible assets, accumulated amortization	(46,369)	(34,384)
Finite-lived intangible assets, net	$ 121,349	$ 119,223
Weighted-average amortization period (in years}	13 years
LoopNet [Member] | Acquired Trade Names and Other Intangible Assets [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Weighted-average amortization period (in years}	15 years

LONG-TERM DEBT (Details)

LONG-TERM DEBT (Details) - USD ($)	Apr. 03, 2014	Apr. 01, 2014	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Repayments of term loan	$ 310,000,000
Long-term debt				$ 338,339,000
LIBOR period (in months)	1 month
Default interest rate per annum on overdue amounts (in percent)		2.00%
Credit facility, collateral			The obligations under the 2014 Credit Agreement were guaranteed by all material subsidiaries of the Company and were secured by a lien on substantially all of the assets of the Company and those of its material subsidiaries, in each case subject to certain exceptions, pursuant to security and guarantee documents entered into on the Closing Date.
Term Loan [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity		$ 400,000,000
Term of loan	5 years
Revolving Credit Facility [Member]
Business Acquisition [Line Items]
Debt instrument borrowing capacity		225,000,000
Term of loan	5 years
Line of credit facility, amount outstanding			$ 0	$ 0
Letter of Credit [Member]
Business Acquisition [Line Items]
Long-term debt			$ 200,000
CoStar Group [Member] | Swingline Loan [Member]
Business Acquisition [Line Items]
Revolving credit sub-facility for swing-line loans and issuances of letters of credit		10,000,000
CoStar Group [Member] | Letter of Credit [Member]
Business Acquisition [Line Items]
Revolving credit sub-facility for swing-line loans and issuances of letters of credit		$ 10,000,000
CoStar Group [Member] | Notes Payable to Banks [Member]
Business Acquisition [Line Items]
Annual amortization, first year after closing (in percent)		5.00%
Annual amortization, second year after closing (in percent)		5.00%
Annual amortization, third year after closing (in percent)		5.00%
Annual amortization, fourth year after closing (in percent)		10.00%
Annual amortization, fifth year after closing (in percent)		15.00%
Federal Funds Rate [Member]
Business Acquisition [Line Items]
Basis spread on variable rate	0.50%	0.50%
London Interbank Offered Rate (LIBOR) [Member]
Business Acquisition [Line Items]
Basis spread on variable rate	2.00%
Basis spread on variable rate, one month interest period (in percent)		1.00%
Basis spread on variable rate, per annum (in percent)		1.00%

LONG-TERM DEBT (Details - Coven

LONG-TERM DEBT (Details - Covenant Calculations)	Oct. 19, 2017USD ($)	Apr. 03, 2014	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Apr. 01, 2014
Debt Instrument [Line Items]
Loss on extinguishment of debt			$ 3,788,000	$ 0	$ 0	$ 0	$ 3,788,000	$ 0	$ 0
Maximum first lien secured leverage ratio after eight quarters after closing date										350.00%
Maximum total leverage ratio after eight fiscal quarters										450.00%
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on variable rate		2.00%
Revolving Credit Facility [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 750,000,000
Term of loan	5 years
Debt issuance costs, line of credit	$ 4,000,000
Unamortized debt issuance expense	5,000,000
Loss on extinguishment of debt	$ 4,000,000
Line of credit, covenant compliance, Secured Leverage Ratio	3.50
Line of credit, covenant compliance, Total Leverage Ratio	4.50
Letter of Credit [Member] | 2017 Credit Agreement [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 20,000,000
Interest rate increase (in case of default)	2.00%
Letter of Credit [Member] | 2017 Credit Agreement [Member] | Federal Funds Effective Swap Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%
Letter of Credit [Member] | 2017 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Initial Basis Spread [Member] | Letter of Credit [Member] | 2017 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.25%
Initial Basis Spread One Month LIBOR [Member] | Letter of Credit [Member] | 2017 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.25%

LONG-TERM DEBT (Details - Inter

LONG-TERM DEBT (Details - Interest Expense and Amortization of Debt Issuance Costs) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Interest expense, debt	$ 9,000,000	$ 10,000,000	$ 9,000,000
Amortization of debt issuance costs	2,000,000	3,000,000	3,000,000
Interest paid	6,000,000	7,000,000	$ 6,000,000
Debt issuance costs, net	(4,208,000)	(6,661,000)
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$ 0	$ 0

LONG-TERM DEBT (Details - Compo

LONG-TERM DEBT (Details - Components of Long-Term Debt) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Term loan facility		$ 345,000
Debt issuance costs, net	$ (4,208)	(6,661)
Total debt		338,339
Current maturities of long-term debt		(35,000)
Current debt issuance costs, net		3,134
Total long-term debt, less current portion	$ 0	$ 306,473

INCOME TAXES (Details - Compone

INCOME TAXES (Details - Components for Provision for Income Taxes) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal									$ 41,453	$ 32,198	$ 10,295
State									3,518	3,682	1,503
Foreign									295	76	40
Total current									45,266	35,956	11,838
Deferred:
Federal									(7,917)	12,586	(8,382)
State									4,695	3,014	2,590
Foreign									319	35	0
Total deferred									(2,903)	15,635	(5,792)
Income tax expense, net	$ 4,487	$ 20,990	$ 3,611	$ 13,275	$ 15,948	$ 14,241	$ 10,247	$ 11,155	$ 42,363	$ 51,591	$ 6,046

INCOME TAXES (Details - Compo68

INCOME TAXES (Details - Components of Deferred Tax Assets and Liabilities) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Reserve for bad debts	$ 1,636	$ 2,437
Accrued compensation	6,706	5,562
Stock compensation	10,568	14,268
Net operating losses	25,899	30,319
Accrued reserve and other	1,393	2,097
Unrealized loss on securities	185	326
Deferred rent	6,533	7,814
Deferred gain on the sale of building	4,741	8,166
Total deferred tax assets, prior to valuation allowance	57,661	70,989
Valuation allowance	(13,032)	(8,557)
Total deferred tax assets, net of valuation allowance	44,629	62,432
Deferred tax liabilities:
Prepaids	(1,239)	(1,753)
Depreciation	(6,229)	(13,045)
Intangibles	(43,800)	(58,747)
Total deferred tax liabilities	(51,268)	(73,545)
Net deferred tax assets (liabilities)	$ (6,639)	$ (11,113)

INCOME TAXES (Details - Reconci

INCOME TAXES (Details - Reconciliation of Provision for Income Taxes) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective tax rate reconciliation [Abstract]
Expected federal income tax provision at statutory rate									$ 57,770	$ 47,832	$ 903
State income taxes, net of federal benefit									4,776	3,638	(678)
Foreign income taxes, net effect									(3,540)	(31)	469
Increase (decrease) in valuation allowance									3,624	(103)	1,956
Nondeductible compensation									230	141	574
Nondeductible transaction costs									0	103	229
Meals and entertainment									958	712	1,032
Tax rate changes									(7,340)	283	1,203
Research credits									(20,547)	(920)	0
Excess tax benefit									(7,010)	0	0
Tax reserves									12,646	(150)	71
Other adjustments									796	86	287
Income tax expense, net	$ 4,487	$ 20,990	$ 3,611	$ 13,275	$ 15,948	$ 14,241	$ 10,247	$ 11,155	$ 42,363	$ 51,591	$ 6,046

INCOME TAXES (Details - Other)

INCOME TAXES (Details - Other) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	$ 4,000	$ 1,000
Cash tax benefits resulting in net operating loss carryforward	7,000	5,000	$ 1,000
Research credits	20,547	920	0
Income from U.S. sources	167,000	135,000	2,000
Income from foreign sources	2,000	2,000	1,000
Income taxes paid	41,000	34,000	$ 1,000
Foreign Country [Member]
Operating Loss Carryforwards [Line Items]
Change in valuation allowance	(4,000)	$ 1,000
Net operating loss carryforward	49,000
Domestic Country [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 38,000
Operating loss carryforwards, first expiration date	Dec. 31, 2020
State and Local Jurisdiction [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward	$ 6,000
Operating loss carryforwards, first expiration date	Dec. 31, 2020
Income tax credit carryforward	$ 3,000
Tax Years December 31, 2013 Through December 31, 2017 [Member]
Operating Loss Carryforwards [Line Items]
Research credits	$ 21,000

INCOME TAXES (Details - Unrecog

INCOME TAXES (Details - Unrecognized Tax Benefits) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits [Roll Forward]
Unrecognized tax benefits beginning balance	$ 1,843	$ 7,664	$ 5,749
Increase for current year tax positions	12,620	368	0
Increase for prior year tax positions			1,954
Decrease for prior period tax positions	(34)	(6,115)
Expiration of the statute of limitation for assessment of taxes	(66)	(74)	(39)
Unrecognized tax benefits ending balance	14,363	1,843	7,664
Unrecognized tax benefits that would favorably affect the annual effective tax rate if recognized in future periods	14,000	1,000
Interest and penalties on income taxes recognized	72		83
Interest and penalties on income taxes reversed		416
Interest and penalties accrued on income taxes	$ 205	$ 133	$ 549

INCOME TAXES - Tax Cuts and Job

INCOME TAXES - Tax Cuts and Jobs Act of 2017 (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Tax Cuts And Jobs Act Of 2017, incomplete accounting, provisional income tax expense (benefit)		$ (7,000)
Tax Cuts And Jobs Act Of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional income tax expense (benefit)	$ 400
Tax Cuts And Jobs Act Of 2017, incomplete accounting, change in tax rate, provisional income tax expense (benefit)		$ (7,400)

COMMITMENTS AND CONTINGENCIES73

COMMITMENTS AND CONTINGENCIES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leases Rent Expense [Abstract]
Rent expense for operating leases	$ 26,000	$ 22,000	$ 21,000
Future Minimum Lease Payments [Abstract]
2,018	30,853
2,019	27,925
2,020	26,136
2,021	24,544
2,022	22,883
Thereafter	46,061
Total future minimum lease payments	$ 178,402

SEGMENT REPORTING (Details)

SEGMENT REPORTING (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)operating_segments	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Segment Reporting Information [Line Items]
Number of business segments (in segments) | operating_segments									2
Segment Reporting Information, Revenue [Abstract]
Revenues	$ 253,991	$ 247,533	$ 237,153	$ 226,553	$ 218,311	$ 212,711	$ 206,869	$ 199,739	$ 965,230	$ 837,630	$ 711,764
Reconciliation of EBITDA to net income [Abstract]
Net income (loss)	44,204	34,177	22,184	22,130	29,573	23,201	15,576	16,721	122,695	85,071	(3,465)
Amortization of acquired intangible assets in cost of revenues									19,707	22,819	30,077
Amortization of acquired intangible assets in operating expenses									17,684	22,731	27,931
Depreciation and other amortization									26,252	24,615	20,524
Interest and other income	(2,455)	(555)	(605)	(429)	(1,186)	(344)	(159)	(84)	(4,044)	(1,773)	(537)
Interest and other expense	734	2,901	2,693	2,686	2,554	2,498	2,455	2,509	9,014	10,016	9,411
Loss on debt extinguishment	3,788	0	0	0					3,788	0	0
Income tax expense, net	$ 4,487	$ 20,990	$ 3,611	$ 13,275	$ 15,948	$ 14,241	$ 10,247	$ 11,155	42,363	51,591	6,046
EBITDA									237,459	215,070	89,987
International [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues									30,808	28,178	25,232
Operating Segments [Member] | North America [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues									934,464	809,492	686,573
Reconciliation of EBITDA to net income [Abstract]
Net income (loss) before interest and other income (expense), income taxes, depreciation and amortization (“EBITDA”)									236,906	210,901	87,092
Operating Segments [Member] | International [Member]
Reconciliation of EBITDA to net income [Abstract]
Net income (loss) before interest and other income (expense), income taxes, depreciation and amortization (“EBITDA”)									553	4,169	2,895
Operating Segments [Member] | International [Member] | External Customers [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues									30,766	28,138	25,191
Intersegment Revenue [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues									42	40	41
Intersegment Eliminations [Member]
Segment Reporting Information, Revenue [Abstract]
Revenues									$ (42)	$ (40)	$ (41)

SEGMENT REPORTING, ASSETS AND L

SEGMENT REPORTING, ASSETS AND LIABILITIES (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Property and equipment, net	$ 84,496	$ 87,568
Goodwill	1,283,457	1,254,866	$ 1,252,945
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,873,441	2,185,063
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	222,191	530,850
North America [Member]
Segment Reporting Information [Line Items]
Goodwill	1,253,494	1,227,777	1,227,310
International [Member]
Segment Reporting Information [Line Items]
Goodwill	29,963	27,089	$ 25,635
Operating Segments [Member]
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,873,441	2,185,063
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	222,191	530,850
Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	79,736	84,727
Goodwill	1,253,494	1,227,777
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	2,816,156	2,139,896
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	201,831	520,833
Operating Segments [Member] | International [Member]
Segment Reporting Information [Line Items]
Property and equipment, net	4,760	2,841
Goodwill	29,963	27,089
Reconciliation of operating segment assets to total assets [Abstract]
Total assets	57,285	45,167
Reconciliation of operating segment liabilities to total liabilities [Abstract]
Total liabilities	$ 20,360	$ 10,017

SEGMENT REPORTING SEGMENT REPOR

SEGMENT REPORTING SEGMENT REPORTING, REVENUES BY SERVICES (Details) - USD ($) $ in Thousands

3 Months Ended

12 Months Ended

Dec. 31, 2017

Sep. 30, 2017

Jun. 30, 2017

Mar. 31, 2017

Dec. 31, 2016

Sep. 30, 2016

Jun. 30, 2016

Mar. 31, 2016

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Revenue from External Customer [Line Items]

Revenues

$ 253,991

$ 247,533

$ 237,153

$ 226,553

$ 218,311

$ 212,711

$ 206,869

$ 199,739

$ 965,230

$ 837,630

$ 711,764

CoStar Suite [Member]

Revenue from External Customer [Line Items]

Revenues

463,185

408,456

360,440

[1]

Information services [Member]

Revenue from External Customer [Line Items]

Revenues

72,618

77,178

75,790

[2]

Multifamily [Member]

Revenue from External Customer [Line Items]

Revenues

279,855

224,835

160,630

[3]

Commercial property and land [Member]

Revenue from External Customer [Line Items]

Revenues

$ 149,572

$ 127,161

$ 114,904

[4]

[1]

CoStar Suite is comprised of: CoStar Property Professional, CoStar COMPS Professional, CoStar Tenant; and CoStar Portfolio Strategy.

[2]

Information services is comprised of: LoopNet Premium Searcher; CoStar Real Estate Manager; CoStar Risk Analytics COMPASS; CoStar Investment Analysis Portfolio Maximizer; CoStar Investment Analysis Request; CoStar Brokerage Applications; PROPEX; Grecam; Belbex; and Thomas Daily.

[3]

Multifamily is comprised of Apartments.com, ApartmentFinder.com and ApartmentHomeLiving.com; WestsideRentals.com; Apartamentos.com; and The Screening Pros.

[4]

Commercial property and land is comprised of: LoopNet Premium Lister; LoopLink; CoStar Advertising; BizBuySell and BizQuest; LandsofAmerica,LandAndFarm, and LandWatch; and CoStar Private Sale Network.

STOCKHOLDERS' EQUITY (Details)

STOCKHOLDERS' EQUITY (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Preferred stock
Preferred stock authorized for issuance (in shares)		2,000,000	2,000,000
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01
Common stock
Common stock authorized for issuance (in shares)		60,000,000	60,000,000
Common stock par value (in dollars per share)		$ 0.01	$ 0.01
Equity offering
Equity offering in period, new shares (in shares)	3,317,308
New shares issued, price per share (in dollars per share)	$ 260
Proceeds from equity offering, net of transaction costs	$ 834,000	$ 833,911	$ 0	$ 0
Payments for underwriting expense	$ 29,000

NET INCOME (LOSS) PER SHARE (De

NET INCOME (LOSS) PER SHARE (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income (loss)	$ 44,204	$ 34,177	$ 22,184	$ 22,130	$ 29,573	$ 23,201	$ 15,576	$ 16,721	$ 122,695	$ 85,071	$ (3,465)
Denominator:
Denominator for basic net income (loss) per share — weighted-average outstanding shares									33,200	32,167	31,950
Effect of dilutive securities:
Stock options and restricted stock (in shares)									359	269	0
Denominator for diluted net income (loss) per share — weighted-average outstanding shares									33,559	32,436	31,950
Net income (loss) per share — basic (in dollars per share)	$ 1.24	$ 1.05	$ 0.68	$ 0.69	$ 0.92	$ 0.72	$ 0.48	$ 0.52	$ 3.70	$ 2.64	$ (0.11)
Net income (loss) per share — diluted (in dollars per share)	$ 1.22	$ 1.04	$ 0.68	$ 0.68	$ 0.91	$ 0.72	$ 0.48	$ 0.52	$ 3.66	$ 2.62	$ (0.11)
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									59	60	56
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									87	194
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									58	59	55
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share (in shares)									1	1	1

EMPLOYEE BENEFIT PLANS (Details

EMPLOYEE BENEFIT PLANS (Details for Stock Incentive Plans) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 09, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 39,030	$ 36,349	$ 34,537
Performance service period	3 years
Options Outstanding 1 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	$ 36.73
Range of exercise price, maximum, (in dollars per share)	$ 58.06
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	41,359
Weighted-average remaining contractual life (in years)	3 years 1 month 17 days
Weighted- average exercise price (in dollars per share)	$ 56.64
Options Outstanding 2 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	58.07
Range of exercise price, maximum, (in dollars per share)	$ 59.59
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	40,004
Weighted-average remaining contractual life (in years)	4 years 1 month 21 days
Weighted- average exercise price (in dollars per share)	$ 58.95
Options Outstanding 3 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	59.60
Range of exercise price, maximum, (in dollars per share)	$ 81.19
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	1,000
Weighted-average remaining contractual life (in years)	3 years 5 months 1 day
Weighted- average exercise price (in dollars per share)	$ 60.23
Options Outstanding 4 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	81.20
Range of exercise price, maximum, (in dollars per share)	$ 142.45
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	70,944
Weighted-average remaining contractual life (in years)	5 years 2 months 9 days
Weighted- average exercise price (in dollars per share)	$ 102.16
Options Outstanding 5 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	142.46
Range of exercise price, maximum, (in dollars per share)	$ 188.22
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	76,934
Weighted-average remaining contractual life (in years)	8 years 2 months 9 days
Weighted- average exercise price (in dollars per share)	$ 182.75
Options Outstanding 6 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	188.23
Range of exercise price, maximum, (in dollars per share)	$ 197.37
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	65,802
Weighted-average remaining contractual life (in years)	7 years 2 months 1 day
Weighted- average exercise price (in dollars per share)	$ 193.69
Options Outstanding 7 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	197.38
Range of exercise price, maximum, (in dollars per share)	$ 202.98
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	62,300
Weighted-average remaining contractual life (in years)	6 years 1 month 28 days
Weighted- average exercise price (in dollars per share)	$ 201.04
Options Outstanding 8 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	202.99
Range of exercise price, maximum, (in dollars per share)	$ 204.91
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	95,500
Weighted-average remaining contractual life (in years)	9 years 1 month 28 days
Weighted- average exercise price (in dollars per share)	$ 204.91
Options Outstanding 9 [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Range of exercise price, minimum, (in dollars per share)	36.73
Range of exercise price, maximum, (in dollars per share)	$ 204.91
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range, End of Period [Abstract]
Number of shares (in shares)	453,843
Weighted-average remaining contractual life (in years)	6 years 8 months 1 day
Weighted- average exercise price (in dollars per share)	$ 156.24
Options Exercisable 1 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	41,359
Weighted-average exercise price (in dollars per share)	$ 56.64
Options Exercisable 2 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	40,004
Weighted-average exercise price (in dollars per share)	$ 58.95
Options Exercisable 3 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	1,000
Weighted-average exercise price (in dollars per share)	$ 60.23
Options Exercisable 4 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	70,944
Weighted-average exercise price (in dollars per share)	$ 102.16
Options Exercisable 5 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	21,999
Weighted-average exercise price (in dollars per share)	$ 182.75
Options Exercisable 6 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	40,633
Weighted-average exercise price (in dollars per share)	$ 193.69
Options Exercisable 7 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	62,300
Weighted-average exercise price (in dollars per share)	$ 201.04
Options Exercisable 8 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	0
Weighted-average exercise price (in dollars per share)	$ 0
Options Exercisable 9 [Member]
Options Exercisable [Abstract]
Number of shares (in shares)	278,239
Weighted-average exercise price (in dollars per share)	$ 130.91
CoStar Group, Inc. 2016 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares)	2,000,000
Shares of common stock authorized for issuance under the plan (in shares)	1,450,000
CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grant under the plan (in shares)				0
Shares of common stock authorized for issuance under the plan (in shares)	815,464
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares)	1,199
Granted (in shares)	69
Vested (in shares)	(306)
Canceled (in shares)	0
Unvested restricted stock at end of period (in shares)	962	1,199
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)	$ 182.64
Granted (in dollars per share)	291.82
Vested (in dollars per share)	183.28
Canceled (in dollars per share)	0
Unvested restricted stock at end of period (in dollars per share)	$ 190.27	$ 182.64
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost expected to be recognized in future years	$ 57,000
Weighted-average-period expected to recognize the unrecognized compensation cost (in years)	2 years 2 months 12 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period (in shares)	440,158	400,077	370,179
Granted (in shares)	95,500	82,400	89,500
Exercised (in shares)	(81,815)	(29,285)	(59,602)
Canceled or expired (in shares)		(13,034)
Outstanding at end of period (in shares)	453,843	440,158	400,077
Exercisable at end of period (in shares)	278,239	284,489	220,107
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average exercise price, outstanding at beginning of period (in dollars per share)	$ 132.08	$ 122.30	$ 99.12
Weighted-average exercise price, granted (in dollars per share)	204.91	182.75	193.69
Weighted-average exercise price, exercised (in dollars per share)	83.07	112.78	85.48
Weighted-average exercise price, canceled or expired (in dollars per share)		195.78
Weighted-average exercise price, outstanding at end of period (in dollars per share)	156.24	132.08	122.30
Weighted-average exercise price, exercisable at end of period (in dollars per share)	$ 130.91	$ 100.94	$ 77.63
Weighted-average remaining contract life of options outstanding at end of period	6 years 8 months 1 day
Weighted-average remaining contract life of options exercisable at end of period	5 years 5 months 19 days
Aggregate intrinsic value of options outstanding at end of period	$ 63,861
Aggregate intrinsic value of options exercisable at end of period	46,199
Aggregate intrinsic value of options exercised	$ 13,000	$ 3,000	$ 7,000
Weighted-average grant date fair value of each option granted during the period (in dollars per share)	$ 59.06	$ 54.34	$ 56.53
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (in hundredths)	0.00%	0.00%	0.00%
Expected volatility (in hundredths)	28.00%	31.00%	30.00%
Risk-free interest rate (in hundredths)	2.00%	1.00%	2.00%
Expected life (in years)	5 years	5 years	5 years
Restricted Stock Awards — without Market Condition
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares)	394,712
Granted (in shares)	156,238
Vested (in shares)	(156,240)
Canceled (in shares)	(30,827)
Unvested restricted stock at end of period (in shares)	363,883	394,712
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)	$ 192.38
Granted (in dollars per share)	211.69
Vested (in dollars per share)	177.72
Canceled (in dollars per share)	196.87
Unvested restricted stock at end of period (in dollars per share)	$ 206.59	$ 192.38
Restricted Stock Awards — with Market Condition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense	$ 5,000	$ 3,000	$ 3,000
Compensation cost expected to be recognized in future years	$ 6,000
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Dividend yield (in hundredths)	0.00%	0.00%	0.00%
Expected volatility (in hundredths)	28.00%	28.00%	26.00%
Risk-free interest rate (in hundredths)	2.00%	1.00%	1.00%
Expected life (in years)	3 years	3 years	3 years
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested restricted stock at beginning of period (in shares)	72,480
Granted (in shares)	32,160
Vested (in shares)	(13,477)
Canceled (in shares)	(5,963)
Unvested restricted stock at end of period (in shares)	85,200	72,480
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Additional Disclosures [Abstract]
Unvested restricted stock at beginning of period (in dollars per share)	$ 202.07
Granted (in dollars per share)	218.59	$ 184.97	$ 208.08
Vested (in dollars per share)	216.20
Canceled (in dollars per share)	216.20
Unvested restricted stock at end of period (in dollars per share)	$ 205.08	$ 202.07
Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Granted (in shares)	32,400	25,680	32,160
Minimum [Member] | CoStar Group, Inc. 2016 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	3 years
Minimum [Member] | CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	3 years
Minimum [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period, (in dollars per share)	$ 36.48	$ 36.48	$ 36.48
Granted (in dollars per share)	0	182.75	193.69
Exercised (in dollars per share)	36.48	36.48	36.48
Canceled or expired (in dollars per share)		196.69
Outstanding at end of period, (in dollars per share)	36.73	36.48	36.48
Exercisable at end of period (in dollars per share)	$ 36.73	36.48	36.48
Minimum [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	1 year
Share-based compensation arrangement, award vesting rights, percentage	0.00%
Minimum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	80.00%
Maximum [Member] | CoStar Group, Inc. 2016 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	4 years
Maximum [Member] | CoStar Group, Inc. 2007 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of options and restricted stock grants	4 years
Maximum [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding at beginning of period, (in dollars per share)	$ 201.04	201.04	201.04
Granted (in dollars per share)	204.91	182.75	193.69
Exercised (in dollars per share)	201.04	201.04	201.04
Canceled or expired (in dollars per share)		201.04
Outstanding at end of period, (in dollars per share)	204.91	201.04	201.04
Exercisable at end of period (in dollars per share)	$ 201.04	$ 201.04	$ 201.04
Maximum [Member] | Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	200.00%
Maximum [Member] | Performance Shares [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement, award vesting rights, percentage	120.00%

EMPLOYEE BENEFIT PLANS (Detai80

EMPLOYEE BENEFIT PLANS (Details for Employee 401(k) Plan, Employee Pension Plan, Registered Retirement Savings Plan and Employee Stock Purchase Plan) - USD ($)	Sep. 15, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee 401(k) Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of company match of employee contribution		100.00%	100.00%	100.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)		4.00%	4.00%	4.00%
Company match to employee contributions		$ 10,000,000	$ 9,000,000	$ 8,000,000
Administrative expense		$ 0	0	0
Registered Retirement Savings Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of company match of employee contribution		100.00%
Maximum percentage of employee total compensation matched by employer (in hundredths)		4.00%
Company match to employee contributions		$ 43,000	$ 10,000	$ 40,000
Employee Stock Purchase Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Percentage of purchase price of company's common stock to the market price		90.00%
Increase in shares of common stock issued pursuant to stock plan (in shares)	100,000
Shares available, employee stock purchase plan		80,022	93,812
Shares of Company's common stock purchased during the period (in shares)		13,790	14,735
Foreign Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Maximum percentage of employee total compensation matched by employer (in hundredths)		6.00%	6.00%	6.00%
Company match to employee contributions		$ 380,000	$ 380,000	$ 420,000

QUARTERLY RESULTS (Details)

QUARTERLY RESULTS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Revenues	$ 253,991	$ 247,533	$ 237,153	$ 226,553	$ 218,311	$ 212,711	$ 206,869	$ 199,739	$ 965,230	$ 837,630	$ 711,764
Cost of revenues	58,301	55,483	55,273	51,346	46,013	42,222	42,679	42,900	220,403	173,814	188,885
Gross profit	195,690	192,050	181,880	175,207	172,298	170,489	164,190	156,839	744,827	663,816	522,879
Operating expenses	144,932	134,537	153,997	137,545	125,409	130,893	136,071	126,538	571,011	518,911	511,424
Income from operations	50,758	57,513	27,883	37,662	46,889	39,596	28,119	30,301	173,816	144,905	11,455
Interest and other income	2,455	555	605	429	1,186	344	159	84	4,044	1,773	537
Interest and other expense	(734)	(2,901)	(2,693)	(2,686)	(2,554)	(2,498)	(2,455)	(2,509)	(9,014)	(10,016)	(9,411)
Loss on debt extinguishment	(3,788)	0	0	0					(3,788)	0	0
Income before income taxes	48,691	55,167	25,795	35,405	45,521	37,442	25,823	27,876	165,058	136,662	2,581
Income tax expense	4,487	20,990	3,611	13,275	15,948	14,241	10,247	11,155	42,363	51,591	6,046
Net income (loss)	$ 44,204	$ 34,177	$ 22,184	$ 22,130	$ 29,573	$ 23,201	$ 15,576	$ 16,721	$ 122,695	$ 85,071	$ (3,465)
Net income (loss) per share — basic (in dollars per share)	$ 1.24	$ 1.05	$ 0.68	$ 0.69	$ 0.92	$ 0.72	$ 0.48	$ 0.52	$ 3.70	$ 2.64	$ (0.11)
Net income (loss) per share — diluted (in dollars per share)	$ 1.22	$ 1.04	$ 0.68	$ 0.68	$ 0.91	$ 0.72	$ 0.48	$ 0.52	$ 3.66	$ 2.62	$ (0.11)

SUBSEQUENT EVENTS (Details)

SUBSEQUENT EVENTS (Details) - ForRent, Division Of DE Holdings, Inc. - Subsequent Event [Member] $ in Millions	Feb. 21, 2018USD ($)
Subsequent Event [Line Items]
Total purchase price	$ 385
Cash payment	350
Purchase price, company stock portion	$ 35

Schedule II Valuation and Qua83

Schedule II Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation and Qualifying Accounts [Abstract]
Balance at beginning of year	[1]	$ 6,344	$ 7,478	$ 4,815
Charged to expense	[1]	5,690	7,358	7,002
Charged to other accounts	[1],[2]	0	0	1,470
Write-offs, net of recoveries	[1]	5,565	8,492	5,809
Balance at end of year	[1]	$ 6,469	$ 6,344	$ 7,478
[1]	Additions to the allowance for doubtful accounts are charged to bad debt expense.
[2]	Amounts represent opening balances from acquired businesses.